23 December 2022

Email Deliverability (and Why it Matters)

Email Deliverability

What is Email Deliverability?

Email deliverability is a crucial metric for email marketers. It tells us what percentage of sent emails land in recipients’ inboxes, how many go into spam folders and the number that end up in an undelivered pile on the virtual post room floor.

It goes without saying why this should matter—even the hottest prospects can’t respond to your finely-crafted offer if they don’t receive it!

Email deliverability can also give insight into the quality of your list and tell you when something is fundamentally amiss with the campaign’s contents.

Let’s dive into deliverability, discover why it matters and—more importantly—look at some easy-to-implement strategies to ensure every last email reaches its mark.

Why Email Deliverability Matters

When you launch a new product or want to alert your audience to your hot new offer, chances are that getting your message into your audience’s inbox will be your channel of choice.

But depending on who you listen to, email acceptable deliverability rates for emails are around 75-80%, meaning that in the worst-case scenario, almost a quarter of all digital mail you send has zero chance of being read.

Just imagine the impact on your campaign if 25,000 readers from your 100k-strong list don’t even receive your offer.

It’s not just marketing, either. Poor deliverability rates can cause password reset or other admin emails to go astray or mean that crucial transactional correspondence is missed.

None of these scenarios is good for business. But there’s also a second reason you’ll want to bump those deliverability rates up beyond industry standard: Poor deliverability could be a sign that you’re straying from best practice, which could land you on a spam list or get you blacklisted by email service providers.

What is a Blacklist?

Consistently falling foul of deliverability best practice could see you added to one of the many blacklists of known spam IP adresses and domains. Email service providers (ESPs) constantly check incoming mail against these lists (of which there are around a hundred), to act as a gatekeeper.

Domain-based blacklists contain the domain names

Needless to say, you don’t want to end up on either type, although it’s possible to get de-listed if you can prove you aren’t an habitual offender.

Check out MXtoolbox in our list of deliverability tools later on to find out how you can check if your name is mud.

Deliverability Metrics

Deliverability isn’t a straightforward metric but a collection of several stats that can vary from business to business and from one use case to another.

Marketing emails, for example, will have much lower deliverability (and open rates) than transactional emails sent for day-to-day business.

Here are the most common metrics you’ll want to keep an eye on and benchmark as you begin your journey to improved deliverability. While some of these metrics, like engagement and open rate, are more concerned with what happens after emails are delivered, they can still impact deliverability and are still worth monitoring.

Delivery Rate

Delivery rate is the most critical metric. Fortunately, it’s also reasonably easy to measure.

In simple terms, delivery rate tells you what percentage of emails from a given campaign reached their intended inbox. Most capable campaign management dashboards will save you the maths by showing you a figure off the bat.

However, the delivery rate doesn’t give any clues as to whether emails landed in recipients’ main inboxes or spam folders.

Inbox Placement Rate

Inbox placement rate (IPR) is similar to delivery rate, with the addition of a significant clue that can help us to drill down and start bringing the metrics up.

IPR tells us how many emails landed successfully in the primary inbox instead of spam or junk folders. As such, it’s a crucial stat to watch for when boosting deliverability.

Low IPR could mean that your content is irrelevant or appears to be spam. It could also point to infrastructural issues like poor IP or domain reputation or too many invalid emails on your list.

Spam Placement

Spam placement rate (SPR) is the counterpart to IPR, telling us what percentage of emails landed in junk or spam folders—another one to watch closely!

If SPR is high, it’s likely your content is being flagged by users or internet service providers and could point to issues with your DKIM or SPF setup (more on this later).

Bounce Rate—Hard Vs Soft

Any email that fails to reach its recipient is said to have bounced—and the bounce could be hard or soft.

Soft bounces happen when mailboxes are full, or the receiver is out of town and has set an auto-reply, although it could also mean the mail server is temporarily down. If you encounter a soft bounce, you might have better luck reattempting delivery another time.

Hard bounces can be more challenging to deal with. They happen when your email is sent to a non-existent address and could mean your list is overdue a spring clean to rid it of old and invalid contacts.


Whether it’s an open, a clicked link, or a buy now (yay!), engagement measures the percentage of recipients who interacted with your email.

This metric takes us into the terrain of what happens after emails are successfully delivered. However, it’s still worth mentioning, as high engagement reduces your chances of being labelled a spammer.

Open Rate

No prizes for guessing this one—the open rate measures the percentage of recipients who opened your email. Here’s where compelling subject lines can help!


If your email content is unexpected, irrelevant or annoying, you risk recipients hitting the Report as Spam button and bumping up your complaint rate.

Even the most diligent marketers encounter complainers from time to time, so a single complaint here and there is nothing to worry about. However, a high complaint rate could see you added to a spam list, from where you could expect overall deliverability to plummet.

Spam Trap Rate

Internet service providers and anti-spam groups often set traps to catch spammers. These are fake email addresses that can usually only be obtained by black-hat methods, and the inclusion of spam trap addresses on your list can severely impact your reputation.

While the spam trap rate isn’t easy to track, you can avoid getting caught in spam traps by validating emails and maintaining good list hygiene.

Testing Deliverability

Any decent marketing email platform should give you a whole host of actionable data right out of the box. There are also plenty of techniques and third-party tools to help senders see, measure and improve performance.

Email Seed Lists

Email seed lists are collections of test email addresses that allow marketers to see how different email service providers will react to their campaigns.


A little like a website cookie, a single discreet pixel inserted into the email via a line of code can return actionable data about deliverability, along with other helpful information, such as the type of device used to read the email.

Third-Party Tools

We’ll compare some of the most popular email benchmarking tools later on. They mostly fall under the categories of sandoxes—tools that allow you to send test campaigns—and reputation testing software to alert you to reputational issues.

Authentication, Encryption and Infrastructure

Maintaining Your Rep with Authentication

ESPs run constant checking to assess the reputation of email senders and protect their users from receiving spam.

It’s a complex metric, but most ESPs will allocate a score from 0-100 based on:

Maintaining Your Rep

Failing a reputation check could get you labelled as a spammer, sending your emails straight to the spam box without passing go. Fortunately, there are some simple steps you can take to keep your reputation in good standing and stay out of jail.

We’ll look deeper at how consistency, content and engagement affect deliverability later. But before we do, let’s dive into the often confusing subject of IP address, server and domain reputation, and some of the authentication technologies available to help senders pass this crucial part of ESPs’ reputation checks.

Authentication—SPF, DKIM, WTF?

If you’ve ever opened an email from a familiar sender or address only to discover an irritating and irrelevant email shilling web design services (or perhaps something less wholesome), you’ve been spoofed.

The technique is as old as the internet itself. Here’s how it works:

A spammer wants you to believe you’ve received an email from a popular brand or someone you know.

They change a few details in the email’s header, and boom! You have an email from PayPal or your bank warning you that you need to log in or perform some other action to avoid having your account suspended.

Now, unless you’re the worst type of spammer or scammer, you’d never send emails like these.

Or would you? Here lies a problem for campaign senders:

The marketing email platforms we use to send emails daily perform a similar kind of trickery—using their servers to send out emails from your address.

So how is an ESPs reputation algorithm to tell your campaign from a spoof?

The answer is authentication—the process by which ESPs determine that sent emails really belong to the purported sender.

For many years, sender policy frameworks (SPFs) and DomainKeys identified mail (DKIM) were the only game in town for reliable authentication. However, they’ve recently been joined by two new contenders—domain-based message authentication, reporting, and conformance (DMARC) and brand indicators for message identification (BIMI).

SPF and DKIM authentication go hand-in-hand to check and verify the sender’s ownership of the email address domain and validate the address as bonafide.

While setting up authentication won’t guarantee you a golden ticket into receivers’ primary inboxes, getting it set up correctly is a vital step towards improving inbox placement rates.

Let’s take a look at how they work.


SPF is an email validation system designed to verify the sender’s IP address and prevent spammers from sending spoofed emails from your domain.

They’re set up relatively easily by adding an SPF record to your website’s domain name server (DNS). The record verifies that you are the domain owner and tells the server which web hosts or ESPs may send mail on your behalf.

This information is checked against DNS records and approved or blocked accordingly. A pass increases your email’s chances of appearing in the inbox, while a fail could cause your email to be labelled as spam.


DKIM authentication allows email recipients to check emails were sent and authorised by domain owners by adding an encrypted digital signature to the email header during delivery.

Email clients check for a valid DKIM signature on all incoming mail, which tells them it was sent from the correct domain and hasn’t been tampered with on the way.

DKIM signatures are set up using a dedicated DKIM package to create a public and private key pair. You’ll then need to add additional records to your DNS.


Bringing together the best of SPF and DKIM, and adding an extra layer of security for good measure, DMARC is the most sophisticated authentication method so far.

DMARC fixes the vulnerabilities of SPF and DKIM through identifier alignments—a test to ensure that at least one of the SPF or DKIM-authenticated domains matches the domain in the email’s header address.

The protocol can also issue instructions to servers about what to do when emails fail authentication checks and return a detailed report to the sender about the cause.

To set up DMARC, you’ll first need to have either SPF, DKIM (or ideally both) enabled. From there, it’s a matter of adding DMARC records to your web host’s DNS records.


BIMI is an attempt by major tech players, including LinkedIn, Microsoft, PayPal and others, to make authentication more accessible and maintainable—and across more channels than just email.

The protocol sees brand logos appear alongside emails in the inbox—an instant visual stamp of authenticity for recipients.

To set up BIMI authentication, you must set up DMARC before creating and publishing a BIMI-compatible logo. You can then add a BIMI record to your DNS.

TLS Encryption

Without encryption, there is little email senders can do to ensure their messages aren’t intercepted and tampered with by spammers and scammers during delivery.

This is where transport layer security (TLS) encryption comes in. While there is little evidence to show that encryption is good for deliverability, its increasing popularity hints that TLS is already considered best practice by most major email and hosting providers.

Unlike the other protocols we’ve looked at, TLS is configured server-side with a certificate and encrypted key.

SPF, DKIM, DMARC, BIMI, TLS – Which is Better?

Each of these authentication protocols has its own strengths and weaknesses—you should use all of them!

Email Infrastructure and Deliverability

If you think editing DNS records and installing TLS certificates is a PITA, then the complex process of setting up and maintaining dedicated infrastructure to handle your campaigns may not be for you.

Here’s the point where we should mention our own automated emailer and CRM system, designed specifically for dentists. Leadflo makes DNS configuration and maintenance simple, with built-in TLS certificates and more. Plus, users get full setup support from a dedicated team of developers and deliverability experts.

However, if you decide to go down that route, there are factors you should discuss and get set up with your server admin.

However, you can also use this infrastructure guide as a guide to features to look for (or avoid) when choosing a third-party email campaign sender.

Dedicated Vs Shared IPs

Depending on the volume and frequency of your campaigns, there are two types of IP addresses you’ll want to consider. Each can impact your deliverability in different ways—so choose wisely!

With a dedicated IP, you’ll have a unique IP address used for your campaigns only, which means you won’t encounter throttling or issues caused by senders using the same IP.

However, dedicated IPs often come at a cost. Generally speaking, they are best for marketers who consistently send emails to audiences of 200,000 or more.

You can obtain shared IPs for free. However, it means sharing the risks and benefits with other users. On the downside, unusually high traffic may cause bottlenecks and slow service, and other bad actors in the IP pool could adversely affect your reputation. Conversely, you could also notice a boost to reputation from other higher-rated users.

Using Custom Domains

Tracking links in the body of your email can trigger the spam filters of some ESPs. You can get around this by creating a custom domain to track engagement. Custom domains are only visible to email readers if they click or hover on links in the body of your email.

Email Content and Deliverability

It’s Probably Canned Cooked Pork

So far, all the talk of MX records, DNS and TLS probably sounds like the kind of techie stuff marketers usually avoid.

But here’s something marketers can act on: email content—everything from subject lines to body copy, imagery and design—can also impact deliverability rates.

ESPs spam and junk filters are powered by algorithms that we mortal marketers poorly understand. That said, if your email walks like spam and talks like spam, it has more chance of triggering a filter.

Here are some simple steps marketers can take to ensure email content doesn’t trigger spam filters.

Spam Keywords—Subject Lines & Email Body

Your subject lines impact deliverability as well as open rates. While you’ll want to inject some urgency into your subject line, adding too much could make your message indistinguishable from spam.

You should avoid writing in full caps and avoid misleading terms like RE or FW that try to fool the reader into believing it’s a personal email.

Say Hello

Let new subscribers know what they can expect to receive after signing up to your list. Even better—send them a warm thanks and tell them exactly when they can expect to receive their first email from you. You could also give readers the option to update their preferences if you have a preference centre. Your welcome email is an excellent way to build trust early on and keep future complaints to a minimum.

Keep it Brief

You may already know that emails that read like War and Peace have a lower chance of keeping readers engaged and converting. Long emails may also trigger spam filters, so it’s best to keep emails as brief as possible from a deliverability standpoint, too.

Be Relevant

If your audience signs up for news and offers about ice cream, don’t send them info about plasma TVs. Although this may sound obvious, sending irrelevant news or offers puts you at risk of readers unsubscribing, making a complaint—or worse—reporting you as spam.

Be Personal

High open rates improve your overall deliverability, and you can improve open rates by personalising your subject line with recipients’ names.

But don’t forget to also personalise the ‘from’ field. A name like ‘Dave from The Fresh’ will likely encourage more opens than a generic name like Dave Jones or an obvious admin account address like Davejones1234.

Be Mobile-Friendly

Around half of all email is now read on mobile devices. So if your email design is only optimised for desktop and tablet, readability will suffer, complaints and unsubscribes will rise, and deliverability will plummet.

In addition, poor mobile optimisation could spark complaints if vital components of your email, such as the unsubscribe link, are impossible to find.

Be Consistent

As a marketer, you are probably already too keenly aware of why sticking to brand guidelines is vital. If your graphics, font, tone or other elements aren’t consistent, you run the risk that readers won’t recognise you or—worse—could mistake you for a spoofer.

Take it Easy with Links

You can’t convert without including a link to your product, service or offer. But adding too many links to your email can trigger spam filters and be detrimental to deliverability. (This includes tracking links!) So, as a rule of thumb, keep links in your email to one or two.

Watch Out for URL Shorteners

URL shorteners can help turn mammoth URLs into appealing, clickable links. But they also mimic a favourite spammers’ hallmark by masking the link’s actual address. Avoid to steer clear of spam filters.

Plain Text is Your Friend

Plain text emails? Really? Sure, they don’t look great but creating a plain text version of your email is best practice since some readers prefer to read them that way for accessibility purposes. Creating readable plain-text emails is a sure way to ensure that anyone who wants to read your email can do so.

Text-to-Image Ratio Matters

It’s thought that the image-to-text ratio doesn’t have the same impact on deliverability that it once did. Nevertheless, most of those in the know recommend a 60:40 text-image split, although this is less likely to affect things if you have all your other deliverability ducks in a row.

Let Readers Reply

Sending mail from a do-not-reply address sends the wrong message to your audience. It tells readers you are unapproachable and don’t want to hear from them, so you’ve already broken a golden rule of marketing by passing up on the opportunity to build a closer relationship.

Most spam filters are designed to filter no-reply addresses out, but if that’s not reason enough to prevent you from using one, consider that using a no-reply address could land you on the wrong side of GDPR compliance.

Use Branded Emails

Sending a campaign from a personal email address is potentially disastrous from a branding standpoint—and just as bad for deliverability. Using a personal email address will trigger spam filters and recipients into labelling your communication as spam.

More Deliverability Tips

With your content now on-point, don’t forget to also follow these strategies for maintaining list hygiene and staying off the blacklists.

Let Them Go

Marketers can’t help but take unsubscribes to heart, but it’s OK! Don’t be tempted to hide your unsubscribe link to avoid a broken heart—this will only encourage readers to mark your mail as spam. Plus, the EU and the US have set out tight legislation on including unsubscribe links in marketing emails, and non-compliance could see you landing hefty fines.

Segment Your Lists

We’ve already seen how relevance can impact open and engagement rates, but when you’re a big brand or company that offers many products or services, how can you be sure that readers always receive relevant emails?

The answer is segmentation—dividing your list by common interests. In dental terms, this might mean keeping your general patients and cosmetic leads on two separate lists. From there, it’s easy to make sure each group only ever receives relevant news or offers.

Consistent Volume

Sending regularly to a schedule means recipients know when to expect your emails, so it’s great for open rates. Consistency will also keep you in good standing with ESPs, so while there may be busy times that you may need to send more emails than usual, you should stick to a consistent schedule where possible.

Stay Clean

When you send messages to invalid or inactive email addresses, those messages are more likely to bounce and send your overall deliverability rate into the dirt. Additionally, sending messages to addresses that are no longer in use can harm your sender’s reputation. Make a habit of cleaning your list regularly and removing inactive addresses or those that bounce consistently.

Verify Your List

Asking subscribers to verify their subscriptions by clicking a double opt-in link after subscribing is a surefire way of ensuring the addresses on your list are active and valid and reducing your bounce rate.

You can build in verification to your ongoing list hygiene strategy by using a tool to identify inactive addresses or those with low engagement.

Avoid Complaints

Nothing damages your sender rep quite like a gaggle of complaints. If readers regularly mark your emails as spam, you should take a long hard look at your email content and follow the steps in the Email Content and Deliverability guide later on.

Build Naturally

Few things are more irritating than having your inbox cluttered with unsolicited emails. But if you’re using a purchased email list, that’s exactly what your emails will be. You’ll receive much lower complaint rates and an improved rep from a verified list you built yourself.

Don’t Get Trapped

If you’re using a crawler to harvest email addresses from the web, you’ll run into a spam trap sooner or later. These are fake email addresses published by ESPs in obscure corners of the net with the sole purpose of ending up on a harvested list. Sending just one or two emails to a spam trap address could see you blacklisted.

Be Warm

ESPs are always on the watch for suspicious activity—like brand-new addresses sending out bulk campaigns. As a rule, you shouldn’t use a new email address to send large campaigns. Instead, start slowly by ‘warming’ your audience (and your email account) with a few emails to get conversations started and build volume from there.

If you have no choice but to send a bulk campaign from a new address, try an email warmup service to help you establish a reputation before campaigning.

Stay Within Your Quota

Your ESP may limit the volume of emails you can send. Going above your quota will cause emails above your monthly limit to bounce. Worse, recipients’ ESPs may view your emails from your domain as undeliverable and mark them as spam in the future.

Monitor Your Complaints

There are a couple of ways to figure out when readers report your campaigns as spam, so that you can remove them from your list. The simplest way is to setup a dedicated inbox to forward complaints to—usually set up as part of your email header.
You could also set up a feedback loop (FBL) if your ESP supports them. The FBL forwards a notification to senders each time someone marks an email as spam. However, it’s up to you the sender to register with FBL services for each provider.

Whichever route you choose to go down, you’ll need to keep a close eye on your reporting inbox for complaints and remove any disgruntled subscribers from your list

Email Deliverability Tools – A Roundup

There are many third-party tools available to help you improve deliverability. When choosing a service, selecting one that will help you improve your chosen metric is essential.

You should also ensure the service uses reputable IP addresses and credentials that won’t harm your reputation.

Here are a few of our recommendations:

DKIM Validator

If you want to improve your email deliverability, check out DKIM Validator – a free and easy-to-use tool that can help you with your authentication.

With DKIM Validator, you can quickly and easily verify that your SPF and DKIM are set up correctly, and it even gives you a spam score for your email content. So you can be well on your way to better deliverability and happier recipients in just a few clicks.


Are your emails landing in the inbox or the spam folder? Find out with Glockapps. With this powerful tool, you can test your email deliverability across different email service providers and get a complete report on your subject line, email body, authentications, and blacklist status.

And if you upgrade to the premium plan, you’ll have access to even more advanced features and detailed analysis.


This handy tool lets you check your email for spammy content, giving you a spam score and providing detailed feedback on how to improve. Simply email the address provided and let ISnotSPAM do the rest.

The tool even gives you a checklist of common issues that can cause your emails to be flagged as spam so that you can avoid them in the future.


This free tool is a simple yet powerful way to get detailed feedback on your email. Just send your email to the address provided by Mail-Tester, and they’ll analyse it for you and provide a comprehensive report on your content and authentications. It’s a great way to ensure your emails are in top shape and ready to deliver.

Mailtrap Email API

With this powerful tool, you can quickly troubleshoot and debug any sending issues, ensuring that your emails always reach their destination. And with an impressive send volume of ~10,000 emails per second, you can be confident that your emails will be delivered quickly and efficiently.


With Mxtoolbox, you can efficiently perform a wide range of tests to assess your domain health and DNS, and it’s also a popular choice for testing your blacklist score.


SendCheckIt is a subject line testing tool that helps you improve your email deliverability. With SendCheckIt, you can easily check the spam score of your subject line and get suggestions on how to improve it.


Zerobounce offers tools to detect potential server issues before sending and perform a test to predict whether emails will go into primary inboxes or spam folders.

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