Email Marketing Deliverability Bible for eCommerce Stores | UPDATED 2024

Email Marketing Deliverability Bible for eCommerce Stores | UPDATED 2024
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[UPDATED 2024] Preface:

Adam: Email deliverability is the most important yet grossly neglected element of email marketing success. In this article, we’ll delve into what it is, why it’s important, and what you can do to maximise your sender reputation.

This article was created in deep collaboration with Nick Koreck, CEO at 360Inbox.
Nick is, for me, the highest authority in the field of email deliverability and the most knowledgeable person I know on the subject.

Pre-warning: there are some technical, geeky parts to this article. It’s a deviation from my normal style of writing but it’s critical that you take the time to read and assimilate the information as it’s important. It’s arguably more important than anything I’ve ever written, as all the strategy I love to write about counts for nothing unless you first understand the art and science of deliverability, and the complexities behind reaching the inbox successfully.

Before we begin, if you enjoy this article and find it useful, be sure to follow Nick on LinkedIn, and check-out 360Inbox - their deliverability tools and solutions are working wonders for our more high-end clients; helping them to monitor their inbox placement, whilst improving deliverability to boost reach and revenue across their email program.


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  • What is deliverability?
  • What deliverability is not
  • Why deliverability matters
  • ROI and uplift from optimising deliverability
  • Factors that affect deliverability
  • Sending infrastructure
  • Technical setup and authentication 
  • Recipient engagement
  • Content
  • Negative intent signals to be aware of
  • Setting up your Dedicated custom sending domain
  • Domain vs. Sub-Domain
  • Dedicated IP vs. Shared IPs
  • Google Postmaster
  • Microsoft SNDS
  • About 360Inbox

Section 1: Introduction to Email Deliverability

What is Deliverability?

Nick: “The term “Deliverability” refers to the process of sending emails to successfully reach the intended recipient's inbox. The inbox is the intended destination, but for a variety of reasons, mailings can be routed to the spam folder or blocked by the receiving mailbox-providers (MBPs) or internet service providers (ISPs).

Deliverability rate is calculated as the percentage of the total number of messages sent that successfully reach the recipient's inbox. For example, if you send 1,000 emails and 800 are successfully delivered to the recipient's inbox, and 20% are filtered to spam, your deliverability rate would be 80%.

For others, deliverability cannot be concluded as a singular metric. To understand a more comprehensive view of deliverability, marketers review various signals and metrics, and test continuously in an attempt to understand what impacts the mailbox providers’ spam filters or inbox-placement algorithm. We’ll get into what these signals and metrics are shortly, but be aware that it’s rarely one variable that causes an issue, since the algorithm is fed by numerous data points; a weighted mix of human behaviours and technical indicators.

The goal for marketers is to understand how our email marketing activities affect deliverability. The first step is often to test and monitor where your emails are routed. With this knowledge, senders should review and make technical, data, and strategic changes to improve sender reputation, and reach the inbox across as many relevant mailbox-providers as possible.

What Deliverability is Not

Nick: “Marketers often confuse deliverability rate and delivery-rate, which measure different things. Delivery-rate only measures whether the email was accepted by the recipient's email server, but does not provide information about the email's actual deliverability to the inbox, spam/junk folder, or other placement.

Deliverability is also not about “gaming the system” or hacks to shift from emails from the Promotions Tab to the Primary Tab.  It’s about the ability to deliver emails to subscriber’s inboxes, which is all about good emails, good data, and good sending practices.

Why deliverability matters

Nick weighs in on some heavy stats here to drive home the point on deliverability:

On average, 1 in 5 emails are filtered to the spam folder or blocked - and that's a huge missed opportunity, affecting even the most seasoned marketers.

Why deliverability matters is therefore quite simple: if an email is not visible (due to spam filtering or blocking), it cannot be read or acted upon by the recipient, which hinders communication and impacts the email’s effectiveness. 

That downside risk of not managing deliverability means missing out on potential opens, clicks, conversion, and revenue. Poor deliverability can negatively affect the customer experience and even tarnish brand perception, causing long-tail damage to engagement, retention, and LTV. Delayed delivery, non-delivery, or mailings lost to spam-filtering accelerates churn, user  frustrations, inbound complaints, and rising operating costs.”

Deliverability isn’t just important for marketing programs. Also consider some of these critical business communications you’d expect to receive immediately, but are often impeded by  poor deliverability:

  • Opt-ins
  • Subscription or Account Registration
  • Password Reset
  • Payment, Shipping, and Order Confirmation
  • Order Confirmation
  • e-Tickets

ROI and Uplift from Optimising Deliverability

Adam: Pay close attention to this section and you’ll understand why I’m adamant deliverability needs to be the cornerstone of your email marketing program before anything else.

It’s like laying down bricks on a house you’re going to live in for the next 100 years VS straw. If you skip this section, it doesn’t matter how well you decorate the home, it’ll always be vulnerable to external conditions.

I’ll let Nick take back over from here to school us on the statistics again:

Nick: “Most senders want to understand the uplift from improving deliverability, and there are a few ways to calculate this. Let’s consider the following example:

Example Conditions:

  • # of (intended) Newsletter recipients in a 1x send-out = 1,000,000

  • What % of the 1-million emails are delivered successfully to the inbox? Let’s assume the average of 80%.  *If you don’t know, you can test your “inbox placement” across 100+ ISP-domains and filters with our deliverability tools.

  • For simplicity, let’s assume an average aggregate uOR of 25% and uCTR of 5% (or 20% uCTOR).

  • Let’s also assume a simple average Conversion-rate of 3% and a $50 AOV.

Now for some quick maths:

  • 800,000 (users who received the email in their inbox) * 25% uOR = 200,000 unique opens (let’s assume these are organic opens, and not from cached pixels or Apple MPP).

  • 5% (uCTR) of the 800k inbox recipients clicked = 40,000 unique clicks

  • Apply our Conversion-rRate * AOV → [(40,000 * 0.03) * ($50)] = $60,000
    *assumed email revenue baseline

Example #1 – assume we were able to improve overall deliverability rate by 1%

  • 10,000 more users received the email to their inbox (800k → 810k)
  • 2,500 more opens (200k → 202.5k)
  • 500 more clicks (40k → 40.5k)
  • $750 in additional revenue ($60,000 → $60,750)

Example #2 – assume we were able to improve overall deliverability rate by 10%

  • 100,000 more users received the email to their inbox (800k → 900k)
  • 25,000 more opens (200k → 225k)
  • 5,000 more clicks (40k → 45k)
  • $7,500 in additional revenue ($60,000 → $67,500)

Example #3 – assume we were able to improve overall deliverability rate by 20%

  • 200,000 more users received the email to their inbox (800k → 1MM)
  • 50,000 more opens (200k → 250k)
  • 10,000 more clicks (40k → 50k)
  • $15,000 in additional revenue ($60,000 → $75,000)

Example #4 – assume overall deliverability worsens by 10%

  • 100,000 fewer users received the email to their inbox (800k → 700k)
  • 25,000 fewer opens (200k → 175k)
  • 5,000 fewer clicks (40k → 35k)
  • $7,500 loss in revenue ($60,000 → $52,500)

Now, consider all the other email communications you send per month or per year, and run the calculations again. The amount of reach and revenue left on the table can be painfully astonishing! 

Other considerations:

  • Good deliverability should be the baseline; therefore, you should consider prioritising deliverability optimization before other email test activities. For example, if you focused on A/B subject-line testing (in an attempt to boost OR%), whilst deliverability remained poor, the impact of those subject-line tests/actions may be negligible and misleading if many of your emails are being blocked or filtered to the spam folder.

  • If you increase your subscriber-base growth and/or send-frequency, it may further compound the ROI uplift.

  • When deliverability increases, an improvement in open-rates may also follow, since better inbox-visibility may increase the probability of future open activity.
  • Deliverability optimization should be considered a top-of-funnel optimization because the impact can be so significant. First, understand your status-quo deliverability rate as a baseline, and measure this across the major ISPs.. Then as you make adjustments to improve deliverability, measure the uplift from the status-quo.

Section 2: Deliverability Reputation

“Sender Reputation” is a simplified term used to define how mailbox-providers assign a score or judge a sender’s reputation based on the quality of their sending patterns and practices, response behaviours, and many other factors. Every mailbox-provider considers a different mix of variables and data-points. Mailbox-providers also use algorithms to monitor a complex set of data-points, assessing the risk of IP-addresses delivering unwanted emails to their users. Ultimately, they need a mechanism to determine whether their servers should accept and deliver email to the inbox, filter it to the spam/junk folder, quarantine it, or reject it outright.

A high sender reputation may indicate the sender is following ‘best practices’ like sending authenticated emails to consented opt-in users; deemed trustworthy and sends high-quality, relevant emails that recipients are interested in. A low sender reputation may indicate that the sender has a (recent or long) history of poor sending practices; sending unauthenticated, poor-quality, irrelevant, or spammy emails to non-consenting recipients.

In short, the higher the reputation, the more likely your mailings will reach the inbox. Lower reputation scores may result in delivery-delays, spam-filtering, or blocking.

Nick explains: “To understand sender reputation, we analyse different signals that ISPs share with us in a variety of ways (SMTP response, Postmaster Tools, Feedback Loops, etc.) These signals convey information and instructions for email senders. By looking in the right places, we can understand if mailings are being delivered, or why they’re blocked, delayed, or spam-filtered.

Most ISPs will provide Delivery signals, using different SMTP classifications (codes, reason/response) about the delivery or non-delivery of your email. Understanding whether your email is filtered to inbox or spam requires additional signals, like Sender Reputation, Engagement, Authentication, etc.

Reputation can be measured on IP-level, Domain-level, Authentication-level, and more. Each of these considers a different mix and weighting of variables, which culminates the overall deliverability reputation. There is a long list of factors to consider, but we’ll focus on the main ones driving reputation:”

Some ISPs provide senders with more transparency or signals than others. Some have active postmasters whom you contact for help, whilst others have tools that provide additional signals. Google Postmaster (GPT) is a good example - they provide a variety of signals about your sending reputation at Gmail, and how their filters identify if your email could be spam or not, using Bad, Low, Medium, or High reputation indicators. Key factors include recipient engagement and sender authentication (SPF, DKIM, and DMARC status). They also provide spam-complaint rates, which signal whether or not you follow good unsubscribe practices, and send to subscribers who want your content.

The example below from GPT shows IP and Domain Reputation of a sender monitored over the last 90 days. This client was experiencing 100% spam-filtering at Gmail, which affected 56% of their audience. They were able to improve their sender reputation in less than two months, and are now 100% inboxing. Source data provide by Gmail / Google Postmaster (URL)

Measuring sender reputation using GPT and other signals can help us to understand how our sending practices are interpreted by the mailbox-provider’s algorithms. Senders should use this data to measure and correlate changes in their sender reputation with changes in their email sending practices.

This next section will cover the key factors that affect your sender reputation as well as how to build strong inbox placement with the ISPs.

Sending Infrastructure

Before sending a single email, it’s important to review and optimise your sending infrastructure.

Nick: “The setup and design of your email infrastructure has to fit your current (and future) business needs. We encounter many senders who have outgrown (not the tool, per se), but their sending architecture. Perhaps they are still using the default technical setup, with minimal authentication. Others are using too many services on a root-domain, with conflicting or missing technical records, and mixed reputation. Volume growth, market expansion, brand acquisition, and sender maturity can also affect the need for changes to the sending architecture or technical setup.

Designing the sending infrastructure is usually the first step for any new program (for both new senders and those migrating from existing platforms). This usually involves scoping objectives, volumes, regions, and understanding the type of email program they want to run. In some cases, it’s necessary to restructure the IP/domain structure for future success.

This step is often overlooked because some ESPs provide senders with a choice between dedicated-IP or shared-pool, without a full understanding of their business, sending practices, etc.

Why does this matter?

Nick: The right ‘fit’ of sending infrastructure plays a big role in the success of your program. Your sending practices, volumes/patterns, data-quality, engagement, etc. are all important factors to consider.

This is one of the most commonly debated topics in deliverability since there are so many factors and trade-offs:

  • Active list size 
  • Frequency of sending
  • Quality of data
  • Engagement
  • Security Needs
  • Type of Content 
  • Reputation Management
  • Sending Practices & Accountability
  • Cost

Here are some practical questions to consider:

  • Is your list sufficient in size and send-frequency to warm-up IP reputation?

  • Is your audience engaged enough to build a positive domain reputation? This can be affected by opt-in practices, list quality, content relevancy, sending practices, etc.

  • What are the risk and technical trade-offs between dedicated sending IP vs. shared-IP pools? Could your program be positively or negatively affected by external senders over a shared-IP pool? Are you an accountable sender who follows good practices to maintain your own reputation over a dedicated IP?

  • Is there a need to separate sender reputation of a domain, brand, program, or geo-region from one another?

  • Are there any security concerns, legal requirements, or business critical needs that may require dedicated sending infrastructure (dedicated or shared MTAs; cloud vs. on-premise; dedicated IPs, etc.)?

ESPs are meant to be responsible for who sends from their infrastructure, and many have policies against poor and unsolicited sending practices. But does this mean the ESP is responsible for deliverability, or is the sender?  It’s easy to blame the ESP for deliverability issues, but the answer is both, and most of the accountability falls on the sender.

However, keep in mind, IP-pools are a bit like neighbourhoods (good, fair, bad, dangerous), and some are managed better than others. It helps to know what neighbourhood you’re in, and who your neighbours are. Are you a good sender in a bad neighbourhood?  Would moving to a new IP-pool or dedicated IP help your deliverability?  Monitoring the data-quality and reputation across shared-IPs can help you evaluate this.

We’ll go over Nick’s recommendations on using a Dedicated vs. Shared IP-pool in a later chapter.

Technical Setup and Authentication

There’s a lot of technical acronyms in this section. You’ll need to be aware of each and keep this as a bookmark for future reference, as it’s likely they’ll start popping up a lot now that you’re focused on optimising your deliverability.

Most important Authentication Records explained:

  • SPF (Sender Policy Framework) is a form of authentication through identifying the IP-address, IP-ranges, and mail-servers approved to send email from your domain. It verifies that the messages coming from a domain in the mail-from are sent by IPs authorised by that domain.

    SPF is an important validation protocol to prevent spammers/phishing/spoofing from impersonating your sender-identity or brand via your domains. Anyone could spoof a sender-address (eg., but the underlying technical record could prove that I don’t own the domain, nor do I send from an IP range that is permitted to send emails via the legitimate domain. If this happened and SPF ‘failed’, the mailbox providers could protect end-users by rejecting the mailing or filtering it to the spam folder.

  • DKIM (DomainKeys Identified Mail) supports a form of validation that requires a public and private key. This adds a digital signature to email messages, which mailbox providers use to decrypt the signature and ensure the email message wasn’t changed or corrupted during transmission.

  • DMARC (Domain-based Message Authentication, Reporting & Conformance) is a protocol that uses SPF and DKIM to determine the authenticity of an email. DMARC helps to identify the intended sender from threats, fraud, and spoofing attempts. It allows a brand to control who’s using their domain, by publishing a technical policy that requests action on messages that fail authentication (none, quarantine, reject). It’s also the prerequisite for BIMI.

  • BIMI is a digital certificate using your brand’s logo, visible in the mailbox. The idea is that emails sent from your domain (with DMARC authentication passing) tells mailbox providers and recipients that this email was sent from the intended brand. This concept of verification ownership helps with trademarking, but also builds brand recognition and visibility within the inbox, and trust between the sender and recipient.

  • rDNS refers to the reverse domain name system which links an IP to your domain, the same way an A-record points your domain to an IP address. This can also be setup as a PTR-record, which checks that the mail-server sending the email matches the IP-address that it claims to be from. This type of ‘alignment’ has become increasingly important for mailbox providers (especially for German-based ISPs). When possible, it’s recommended to set up a custom-branded rDNS record, where the IP reverts to your custom-domain, and not the ESP’s default domain.

  • MX-Record or Domain Authentication - your sending address should also be able to receive email, not only send email. Therefore, it must have a valid MX (mail exchange) record; otherwise, some mailbox-providers will block your email. You can either add an MX record to the domain, and/or create a mailbox for the domain and sender-address → if is the sender-address, it must be able to receive emails (even if you define a different reply-to address, or don’t intend to receive emails here).

Recipient Engagement

Marketers, pay attention to this section. It’s a call-to-action to pay attention to the quality of the content you’re delivering to recipients.

Nick: “Engagement plays such a big role in deliverability, but what does engagement really mean? It cannot be deprecated to a single metric, but rather, a more holistic view or story about how users are interacting with your emails. We can categorise this further by ‘positive engagement’ and ‘negative engagement’ signals. Depending on the metric, ‘no action’ could also be perceived as positive or negative engagement.

Mailbox providers use algorithms, machine learning, and AI to evaluate these data points, and measure them along various rolling time series (both recent and long-term history). They consider both the individual subscriber’s behaviour (profile level), and the aggregate behaviour across all recipients (macro level), to determine how to handle email delivery and mailbox placement.

Let’s look at some common engagement signals:

  • Total opens; unique opens (# and %)
  • Total clicks; unique clicks (# and %)
  • Opened → , no click (read vs. dismissed)
  • Opened → , clicked
  • Repeated open/click
  • Ignored / no-open
  • Opened → , deleted
  • Deleted /, no-open
  • Moved from Spam-folder to Inbox
  • Moved from Inbox to Spam-folder
  • Move to another tab/folder
  • Marked as important, starred, saved to contact-list
  • Reported as spam or junk
  • Reported as phishing
  • Replied-to
  • Forwarded-to
  • Unsubscribe rate
  • Unsubscribe click as % of clicks

We like to think of email as having some kind of ‘DNA’ - comprised of unique and common elements within each email, often shared across the wider program. Each email has some reputation behaviour associated with it, and both positive and negative engagement can be mapped or ‘fingerprinted’ to various components of your email - including, but not limited to, your sending domain, authentication, image-URLs, link-URLs, and even your HTML template and layout. 

We remind marketers to think more broadly about the engagement of their campaigns, across their entire program, and across various time series.  One-off poor performance issues doesn’t per se mean it was filtered to spam, and one-off spam issues don’t necessarily mean the issue will continue - that all depends on a huge mix of factors, how persistent the campaign performs positively or negatively, and the impact it has on total volume and engagement over the IP/domain. A sender with a long history of poor sending behaviour and issues could require more time to rebuild their sender reputation than an occasional offender. The time-to-resolve depends on the mix and severity of issues, and the timeframe in which they occurred.

Your contact strategy and sending practices can therefore impact deliverability, and finding the right balance is key. This could mean communicating more frequently to users who react positively, communicating less frequently to users who don’t engage, or the use of different messaging. The so-called ‘sweet spot’ for engagement is before the point of diminishing returns -whereby engagement remains sufficiently consistent, at levels that reinforce positive sender reputation, without causing deliverability issues, all whilst optimising reach and revenue KPIs. Did you get all that? It’s complicated, and this is where CRM strategy and Deliverability come together. The thought process is therefore, to always keep in mind what impact your email activities will have on deliverability, and make changes accordingly.


The best way to bolster positive recipient engagement is with quality content that is relevant to your audience. Nick explains the impact this has on promoting a positive sender reputation to the ISPs:

The HTML code, template, or content itself is often NOT the root issue affecting deliverability; rather, it’s the ‘relevancy’ rooted in your content that matters. Relevancy affects engagement, and engagement is a huge driver of Sender Reputation. If you send irrelevant content, users will become disengaged, unsubscribe, or even complain. Segmentation, cohort clustering, and user-centric targeting are therefore critical to support relevant content delivery.

There are some great development features to enhance engagement through content. Annotations (for Gmail) are code-based customizations for email, allowing senders to create buttons and visual cards within mailbox preview. Using annotations can also help Gmail to highlight your email within the Promotions Tab. Yahoo has something similar with promotions schema, and AMP for email allows senders to create rich engaging components within the email. The future of these content-based customisations is still unclear, whether mail-client support will lead to further adoption. However, the use-cases are quite interesting, and any action that bolsters user-engagement is worth exploring.

How your email renders in the mailbox will vary across client, web/app, and device. This is when HTML coding and styling can affect visibility and click-action of your emails. Image-size can affect load times, depending on the recipient's bandwidth and geo-location in relation to where the images are hosted. Heavy HTML and long-vertical templates can be ‘clipped’ by some mailbox-providers. If the email doesn’t load properly, users may dismiss the message, and this impacts the probability of future open/click - which in-turn affects engagement metrics, and may lead to deliverability issues.”

Adam: I think this is the strongest implication yet that engagement and relevancy is far more important than the tired image vs. text-based email debate.

List Collection & Data Quality

Your email list is one of your most valuable assets. No matter how much you focus on improving activation, preventing churn, or boosting retention, top-of-funnel activities such as list growth and data-quality are still critically important for the longevity of your email program. We know why list growth is important, but what does it really mean to maintain a clean and healthy email list?

The process of how you collect new email subscribers, collect opt-in consent, and where you collect emails from, and where you send email to also matters, not only for deliverability performance, but for legal compliance. The saying “garbage in = garbage out” very much applies here. 

First, Nick explains the wrong way to build an email list:

  • Buying, Renting, or Sharing a List – This involves acquiring a list of user addresses who don’t know your brand, don’t know how you got their email, and never asked for your emails. The laws vary across legal jurisdictions, and there are some grey-zones, but most mailbox providers share similar policies around opt-in practices, consent, and negative signals, and will punish accordingly. These practices are also against the policy agreement for many ESPs. If you’re adamant, consider adjusting your expectations, and separate these leads from your core sending program (different domain / IPs) in order to protect core revenue drivers, sending reputation, and deliverability performance.

  • Email Scraping and Cold Email Outreach – Bots crawling websites for email addresses, software piecing together names and email formats, and burning through sender-addresses and IPs - these are all bad practices, and a quick way to find yourself in the deliverability hell. It’s more common in the United States than in the UK or Europe, but policy changes in America may soon curb this. 

Nick explains how to build and maintain your email list the right way:

  • Attract new subscribers through great content, products, offers, and incentives.

  • Be clear about the language around the opt-in process, and what they will receive from you in return for their email address; consent is key!

  • Collect the opt-in date/time-stamp and source IP-address; you want a clean data record of when a given user signed-up, and what they opted-in to receive.

  • Make it easy for users to subscribe (and unsubscribe), optimise different attention-grabbing forms and collection across the user experience. Don’t blur the line between ‘registration’ and ‘email subscription.’ You can offer an opt-in to various communications and channels within the registration process, but you cannot prevent a user from registering or force them into a communication stream. High spam-complaints and poor engagement are obvious reactions when you surprise users with communications they didn’t agree to receive, and expectations aren’t met.

    Familiarise yourself with the difference between ‘explicit’ consent vs. ‘implicit’ consent. Klaviyo provides a simple overview of this HERE.

  • Understand the subscriber’s lead-source/origin - Invalid and poor performing email addresses often come through a select number of lead sources. Auto-fill, single-sign-on forms, and refer-a-friend programs are also especially problematic, pulling-in expired or secondary email addresses. The bounce rate and engagement will vary across your lead sources, and some may need additional list hygiene, or segmentation to control the impact.

  • Email Verification and List Hygiene - Software that helps identify if a user-address from a known domain is valid, invalid, or unknown. This procedure should be done before sending the first email, preventing unnecessary hard-bounces from affecting your sender reputation. Some verification services can scan for role addresses, malformed domains, and other non-human-like behaviours that can be harmful to your list and deliverability reputation. 360Inbox supports a variety of solutions that help senders analyse and maintain healthier email lists. Most tools support API integration on the form / sign-up level. Klaviyo is currently blocking these integrations with their native sign-up feature; however, most 3rd-party sign-up tools support email verification integration, and can sync this data back to your ESP, enriching your mailing list.

  • Managing Risk - There are many ways to analyse and cohort the behaviours across an email list, but one of the best practices is to evaluate who the riskiest users in your list are, and what impact they have on your email program. Look for users who subscribed many years ago and are unresponsive to email; users who never engaged with email nor other channels, and who never purchased. What percentage of your list do these risker addresses make-up, and how often are they mailed-to? If there are no ‘signs-of-life’, the risk/reward trade-off may be too high to continue contacting them. Deliverability often suffers when the same contact and content strategy are used for all users. Know when to reduce send frequency, when to pause, when to use other channels, and when to retire a user-address from your list.

Negative Intent Signals

Keep a watchful eye on all of these factors as they can adversely impact your sender reputation when the ISPs are pushed too far.

Spam complaints: When a user marks your email as ‘spam’ or ‘junk’ to the ISP. According to Nick, emails marked spam are one of the strongest negative intent signals to the ISPs, indicating real user-friction. You should be aiming to keep complaint-rates below 0.3%, and ideally as low as 0.1%

Bounces: An email bounce is non-delivery to the intended recipient. The returning server (i.e. will provide an SMTP response code/error/reason message back to the sender.

There are two main types of bounces:

  • Soft bounce: email address is valid but was bounced back for various reasons (server was down, network disruption, mailbox full, blocked, etc.)
  • Hard bounce: email address is invalid or doesn’t exist

Klaviyo will automatically suppress hard bounces, while soft bounces need to be managed more carefully (I recommend putting rules in place to limit your sends to soft bounces over time).

You’ll also need to be aware of two terms you’ll hear a lot of in regard to email deliverability, and these are:

Spam Traps: email addresses that shouldn’t be on your mailing list as they don’t belong to a real user. Spam traps are used by ISPs and anti-spam networks to identify bad senders with poor list collection and data hygiene practices. This is why managing consent around opt-in is critical as well as clear segmentation rules to eliminate the risk of damaging your sender reputation.

Blocklistings: these networks help to police the email marketing world of bad senders. Reputable listings such as Spamhaus & Cloudmark share data and reputation signals to both ESPs and ISPs - this helps them to identify suspicious or untrustworthy senders. Getting yourself removed from them is a huge drain on resources and will erode your ability to connect with your customers while you’re in the jailhouse.

  • Data collection & consent
  • Negative intent signals to be aware of
  • Identifying missing reach and inboxing uplift potential

Section 3: Sender Architecture

Earily, we learned about deliverability and the importance of its reputation. Now we’ll be delving more into the technicalities of deliverability optimization.  It’s time to learn how to optimise your sending architecture.

This will have major implications for how you’re received by the mailbox-providers, so at the very least, pay close attention to this section and implement the steps to start improving your deliverability from day one.

Setting up a Dedicated Custom Sending Domain

Nick recommends all brands to use a dedicated custom sending domain. Anybody who uses the default Klaviyo setup (eg. “via”) is practically guaranteed to have worse inbox placement if they don’t optimise this step.

Klaviyo themselves made a huge push for brands to optimise their sending infrastructure in 2021 when a Gmail algorithm update spooked a lot of senders and started leading to reduced ROI.

Example - Klaviyo default setup:

Why is a custom-domain setup important?

  • The default ESP setup (in this case, Klaviyo) will designate a generic Klaviyo domain to your IP that doesn’t align with your personal domain.
  • Default setup will only allow DKIM authentication to pass generically; it will not be authenticated to your actual domain. This means DKIM reputation is again shared on the default domain “” (not optimal).

Nick weighs in on more of what the custom sender domain actually achieves regarding the technicalities:

Using shortcuts and default domains is a thing of the past. Misconfigured alignments are one of the most common issues we find when auditing new clients. When you set up a ‘Dedicated Custom Sender Domain’ in Klaviyo, it allows the DKIM and SPF to be assigned to your domain or subdomain, rather than Klaviyo default domain.

Sending emails from your own branded authentication domain helps you to build a dedicated sender reputation and protects your own domain's reputation from shared senders. It also supports authentication alignment, which is critically important for many ISPs. Alignment means your Envelope-From, Mail-From, and all your branded authentication records use the same domain structure.

Authentication is not enough to guarantee inbox placement, but it’s important for delivery and can reduce the likelihood of your emails being rejected or filtered to the recipient's junk folder. Authentication protocols not only protect email delivery from being compromised (fraud, spoofing), it can help identify and control those threats, and identify you as a legitimate and trustworthy sender (DMARC is specifically good for this).

If you are worried about deliverability using Klaviyo’s default setup, check out 360Inbox. Nick’s team can support setting-up a more advanced setup in Klaviyo for optimised deliverability. Their deliverability tools can help you measure your inbox / spam placement across nearly 100 different ISPs and filters, and measure the impact of changes to your technical setup and sending practices.

Domain vs. Sub-Domain

This is another under-discussed topic that is critical to cover before setting up your sending infrastructure, especially for high-volume senders.

Again, I’m going to let Nick explain this section along with the context to consider.

Nick: This discussion often happens when a sender’s email program matures, when migrating to a new ESP, or when moving to a dedicated IP. So what are the main considerations you need to consider when deciding between a domain vs. sub-domain?

  • Since domain reputation plays a role in the greater scope of sender reputation, sub-domains can be used to define, segment, or silo one mail-stream from another, both from a branding perspective but also for sender reputation.
  • If your root-domain (organisational-domain) is used by other teams or programs across the organisation (which you may or may not control), a sub-domain can help separate the reputation. Marketing programs often use a subdomain, because of their risk to other programs (complaints, spam).
  • The next most common use-case is to separate Transactional mail programs from Marketing / Commercial programs. In part, this is to protect the sender reputation (business-critical and time-sensitive mailings) from other riskier programs.

    There are policies and laws that define how emails should be transported for sending and receiving. The general guidance is that non-commercial vs. commercial email programs should not be routed over the same IP-address or IP-pool. However, the sender has more choice to decide whether the same domain or subdomain should be used, or if different sub-domains should be used to separate these programs. For example:
  • (transactional)
  • (marketing)

  • If too many services use your organisational root-domain, the reputation can become muddled. This makes resolving deliverability issues more complicated, and increases the probability for authentication failure and conflicts to occur. This is especially relevant if you are sharing your domain with 3rd party services (retargeting, loyalty, etc.) which you may have less control over, yet can affect your domain’s sender reputation.

Dedicated IP vs Shared IPs

When to use a Shared IP Pool:

  • Low and infrequent sending-volume (volume < 100k per month)
  • High-volume blasts and infrequent cadence (where as dedicated IPs require more consistency)
  • Sending Maturity (since dedicated IPs require more accountability)
  • Speed of onboarding (since dedicated IPs require more time to warm/ramp-up)

When to use a Dedicated IP Pool:

  • Sending volume and engagement is consistent (volume > 100k per month)
  • Sending maturity is higher (since IP reputation is controlled by you)
  • Easier to identify, diagnose, and remediate deliverability issues (not influenced by external sender’s reputation)
  • Separating brands, programs, and mail-streams (eg. marketing vs. transactional)

Section 4: Deliverability Monitoring and Optimisation

Now that we have a better understanding of email deliverability, the next step is to test, monitor and optimise; otherwise, all the hard work you’ve put into crafting a great email could fall flat.

Given the complexities often involved in setting and optimising your sending infrastructure, it makes sense to continuously test and monitor your email deliverability, and learn what changes to your program affect sender reputation and inbox placement. Resolving deliverability issues will ensure you’re in a strong position to maximise the potential of your email program, boosting ROI long into the future.

We recommend setting up Google Postmaster, since Gmail usually is the top mailbox provider you need to focus on reaching. If you’re experiencing reputation with Gmail, it’s probable you’re experiencing issues with other mailbox-providers as well. More sophisticated senders and advanced accounts should integrate Nick’s 360Inbox deliverability tools for ongoing inbox monitoring and incrementality.

Google Postmaster

What is Google Postmaster and why is it important?

  • Google Postmaster Tool (GPT) is a free online tool provided by Google that provides marketers direct access to Gmail IP and Domain reputation, complaint rates, and authentication diagnostics.  If you send commercial emails to Gmail users, you should use this tool to analyse your Gmail reputation and monitor issues that affect your Gmail deliverability performance.
    *Google Postmaster’s data-feed can be directly integrated with 360Inbox’s deliverability testing and monitoring platforms.

How to Setup Google Postmaster Tools:

  • Postmaster Tools are domain based, and require you to add and verify your authenticated email domain to uniquely identify your email traffic and provide access to your email performance analytics. Here’s how to get started:
  • Go to → Login with a Gmail or G-Suite/Workspace email address that you wish to serve as the account to manage your Postmaster Tools data. *Note, if you have a team that may need access, consider a team-alias or create a new Gmail-account just for Postmaster Tools.
  • Click the big red “+” button; a pop-up prompts you to enter your domain or subdomain, and click “next”.
  • A unique verification TXT record will appear by default that must be added to the DNS of your domain or sub-domain zone, to prove domain ownership.
    Example: google-site-verification=abcdefghijklmnop
  • After publishing the TXT record in your DNS console, return to GPT and hover over the requested domain, or click the 3-dots (“⋮”) icon to the right of the domain, and click “verify”.
  • You can repeat the process to add multiple domains and sub-domains that you own. Now begin monitoring your domain and IP reputation, check authentication results, and review for spam complaint rates.

Microsoft SNDS

  • Microsoft’s Smart Network Data Services (SNDS) is a free service offered by Microsoft. Like other postmaster tools, SNDS provides important data-points that affect sender reputation and email deliverability to Microsoft mailboxes (Hotmail, Outlook, Live, MSN).
    *SNDS data-feed can be directly integrated with 360Inbox’s deliverability testing and monitoring platforms.

About 360Inbox

Nick has spent the last 15 years building start-ups and leading CRM teams, with a strong focus on email deliverability. 360Inbox provides senders with deliverability solutions, including a Deliverability Platform for testing/monitoring/analytics, Email Verification Services, and Deliverability Professional Services.

Their deep partnership with Inbox Monster allows senders to monitor global and regional mailboxes, across nearly 100 different ISP-domains and B2B Filters. 360Inbox offer scalable solutions for most high-volume email senders:

  • Unlimited inbox placement reporting per domain(s)
  • Domain and IP address reputation monitoring
  • Engagement pixel tracking
  • Comprehensive blocklist and spam-trap detection
  • DMARC threat reporting
  • Content filter analysis
  • HTML / Creative rendering services
  • Notification Alerts (email/slack) for threshold detection
  • Google Postmaster & SNDS integrations
  • ESP integrations (SendGrid, SFMC, AWS SES)

Deliverability Monitoring Platform from Inbox Monster

360Inbox offers a Professional Services team to provide deliverability support services, which includes: 

  • Proactive monitoring across all client deliverability KPIs and alerts:
  • Global inbox placement reporting 
  • Spam-trap data; blocklistings 
  • Sender reputation and user complaint metrics 
  • Authentication and DMARC monitoring 
  • Spam and block issue remediation, postmaster outreach, and strategic guidance.

Conclusion: Deliverability is Paramount

We’re closing in on 7,000 words now, so it’s time to wrap things up.

The TL;DR version of this article is this: deliverability is paramount. Without optimising your sending infrastructure and sender reputation, your strategy counts for nothing as you’ll never even reach the intended recipient.

There is a lot of complexity within this article and technical knowledge. At the very least, get a dedicated sending domain setup, the correct authentication, and choose on a dedicated vs. shared IP Pool, before proceeding to ongoing monitoring of your sender reputation with the tools above.

Last but not least, a massive thank you had to go to Nick Koreck at 360Inbox who contributed so much to this deliverability bible. He deserves support, so please be sure to connect with him and work with his team to maximise your inbox placement on an ongoing basis.

More eyeballs on your emails = bigger ROI. Don’t forget this.

Get started with 360Inbox now.

Better deliverability tools to enhance your

  • Inbox placement
  • Reputation motoring
  • Professional services


Adam Kitchen's Monster Email Marketing online training course
If you'd like to learn how to master email deliverability & Klaviyo, our Monster Email Marketing course covers everything you need to know with over 54 video lessons and countless PDF guides and email design examples - OUT NOW.

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