delivery truck with email envelope on the side.

Email Deliverability Guide: Find Your Way to the Inbox


Email deliverability is where email marketing begins. It’s also a complex, multi-faceted topic. But — we’re going to break down the basics in this guide.

Email marketers like to say that the best email in the world has to be opened before it does any good. But even more fundamental, the best email in the world that doesn’t get delivered has no chance at all.

If your email deliverability rates are anywhere below 95%, you’re not just at risk of losing money from missed opportunities, you’re also risking your sender reputation, which can prevent you from being placed in readers’ inboxes at all, or maroon your emails to the spam and promotions folders.

Email deliverability is like an insect trying to weave its way through a spider web of blocklists, spam traps, domain and sender reputations, email authentication protocols, and anti-spam laws.

The insect is your email. And your job, in part, is to persistently clear away all the sticky webs so your message doesn’t get caught before it reaches its recipient. This article will walk you through how to do that by helping you understand the spider webs, so you can successfully deliver your emails to as many customers as possible.

What causes email deliverability problems? 7 Reasons:

Rain, snow, wind, and hail may not affect email deliverability, but many other things can. And just to be clear, email deliverability is not the same thing as inbox placement. That is, you can have an email that is ‘delivered’ but still doesn’t show up in the recipient’s inbox. Some of the problems we’re about to address can lead to inbox placement frustrations as well.

Insight into Spam

1. Your sender reputation

As an email sender, you have a reputation. And that reputation doesn’t depend on how good you look or how much money you donate to charity. If you’re sending emails to spam traps, have a high bounce rate, or are failing to cleanse your list regularly, your sender reputation will fall lower and lower.

Why does that matter? Because if you get labeled as an unreliable email sender, various entities such as spam filters and blocklists will prevent some of your emails from being delivered.

Likewise, if your server/IP address or your domain are labeled as spammers, mailbox providers will start rejecting your emails. We’ll talk later about how these labels get affixed to you, how to shed them if they do, and how to avoid them altogether.

Google Postmaster Tools is one way to learn about your current email reputation and deliverability performance.

2. Hard and soft bounces

Bounces happen when your email is turned back by the mailbox provider. The more bounces you accumulate with each new email you send, the more you put your sender reputation at risk, which could further damage email deliverability.

What is a soft bounce?

A soft bounce is usually the result of a temporary issue. The email address is real, but for some reason, your message can’t be delivered. Common reasons for soft bounces include:

  • The contact’s email inbox is full.
  • The size of the email is too large to be delivered.
  • The contact’s email server is down/offline.
  • Mailbox configuration issues.
  • Failing to meet the email server’s anti-spam, anti-virus, or sender requirements.

You can try re-sending to email addresses that result in soft bounces. But if it continues to happen, those contacts should be removed from your list.

What is a hard bounce?

A hard bounce is the result of a permanent deliverability issue. It usually happens when you send to a non-existent email address. Common reasons for hard bounces include:

  • The email address doesn’t exist.
    • A fake email was used.
    • There was a typo in the email address.
    • It’s an email address for an old employer or school.
  • The domain name doesn’t exist.
  • The email server is blocking your domain or IP address.

You can also end up with hard bounces if you purchase an email list with invalid email addresses. It’s one of many reasons to never purchase email lists.

Remove email addresses that result in hard bounces immediately. These will definitely have a negative impact on email deliverability. There is no reason to try re-sending to contacts that produce a hard bounce.

Most hard bounces, at first, are not directly your fault. But, it is your job to purge them as they occur. Not paying attention to list hygiene is a hallmark of spammers.

The more often you do the things that spammers do by nature, the more likely your IP address or domain will be perceived and labeled as one.

3. ISPs and throttling

One way internet service providers (ISPs) like Verizon and Comcast have learned to identify spam is by the volume of emails sent. Spammers like to send millions and millions of emails — and often do so all at once.

So when you send too many emails at the same time in a way that’s uncharacteristic of your sending patterns, you can be misidentified as a spammer. How do you avoid this? With throttling.

Throttling means you send your emails out in batches instead of all at once. If you have a list of 500,000 emails, you might send out 25,000 at a time, spread out over a few hours. And 25,000 very well could still be too many. You can set up your own email to do this, and some ISPs do it automatically.

The key is to keep the mailbox providers — like Yahoo and Google — and the email service providers (ESPs) happy by respecting their limits. If they have a limit of 10,000 emails at a time, and you try to send 25,000, your emails might be rejected and go unsent. And sometimes, you don’t even know this is happening.

Enterprise-level businesses often hire an email deliverability consultant for this purpose. They can reach out to the mailbox providers, find out their limits, and work with them to confirm that you are a legitimate email sender.

4. Email authentication

Keep your email spam-proof with DKIM, SPF and DMARC email authentications

Here come the acronyms. Don’t say we didn’t warn you. But this is where you get into the technology involved with sorting spam and malicious content from real, wanted emails. There are currently four main email authentication protocols: SPF, DKIM, DMARC, and BIMI. We’ll discuss each in brief.

The purpose of email authentication is to protect inboxes from spam and malware. It’s attempting to confirm that an email sender really is who they say they are.

You as an email sender can create a policy for each form of authentication. You publish that policy in your Domain Name System (DNS) server, which can be read by the mailbox providers, spam filters, and other entities. The DNS will translate your domain name into an IP address so you can be identified. The authenticators attempt to confirm that the IP address associated with your email is the same as the one sending the email.

For example, Amazon is frequently taken advantage of by spammers, who create fake shipping orders and fake invoices. Amazon’s many official email addresses, in theory, should each have their own IP address in their DNS server, and a policy associated with each authentication method. When the IP of a spam email doesn’t match what Amazon’s policy says, the mailbox providers can identify it as spam and refuse to deliver it.

So, if you don’t use email authentication methods, you’re leaving it up to chance if the mailbox providers will correctly identify you as a legitimate email sender.

SPF – Sender Policy Framework

No, this isn’t sunblock. SPF attempts to verify that the email address and the name an email says it’s from really are who they say they are. The mailbox provider looks to validate your SPF before it lets the email be delivered. SPF’s biggest flaw is that it doesn’t work if an email is forwarded, because the new sender is not the company that sent the email. That forwarded email may not get delivered.

Learn more about SPF here

DKIM – Domain Keys Identified Mail

Very different from SPF, the DKIM authentication method lets the sender apply a digital signature of sorts to every email they send out. The recipient’s mailbox can then verify that sender’s identity using the DNS server. Your digital signature is a piece of code, not a literal signature.

The DKIM test guarantees the authenticity of an email sender. However, not all emails that fail the test also fail to get delivered. Some are allowed through anyway, and it’s left to the mailbox provider to quarantine it, let it through, put it in your spam/junk folder, or include a warning in the message that this may be a suspicious email.

See how to test your DKIM and make sense of the information

DMARC — Domain-based Message Authentication, Reporting, and Conformance

This method essentially combines SPF and DKIM and uses both to verify the identity of the email sender. The DMARC policy spells out how the email is authenticated and what the receiving mail server should do if an email violates it.

Authentication gets complicated, but it’s better to be lost in the weeds than stuck in the spider web. Here’s more about DMARC.

BIMI – Brand Indicators for Message Identification
Gif of BIMI in the inbox

If you have a Yahoo email address, you’ve seen BIMI in action. BIMI places the logo of the business that sent the email next to the subject line. The logo confirms that this email really is from that business. It’s a visual verification, but from the email recipient’s perspective.

Since most people don’t know code or even where to find the code for an email, BIMI gives the average email recipient a simple way to verify the authenticity of an email. For big companies like PayPal and Amazon that are frequently exploited by spammers, and for financial institutions that hold sensitive data, BIMI can be a great help for consumers.

You cannot set up BIMI unless you already have the other email authentication methods in place.

Download our free BIMI report!

bimi ebook cover with hermes character

5. Spam complaints

Yet another cause of poor email deliverability is spam complaints. When users mark your email as spam, that’s bad for you. If they’re tired of your emails or don’t know how they got on your list, you want them to unsubscribe, NOT mark you as spam.

Yes, unsubscribing can be a  good thing.

When a subscriber labels your email as spam, the spam filters and blocklists light up with glee and add your email to all sorts of lists you don’t want to be on. Okay, it’s not quite like that. But the more people who label you as spam, the worse your sender reputation gets. That means worse email deliverability.

6. Reader engagement

Yes, open rates matter. Engagement affects deliverability. If you’re getting terrible open rates — like 3% — mailbox providers and other entities will look upon you with great suspicion. Remember, spammers can’t write, and they’re bad at marketing, which is why hardly anyone opens or clicks on their emails.

So, if hardly anyone is engaging with your emails, what’s a good spam filter supposed to think? To them, you look like a spammer, someone who is sending emails to a bunch of people who don’t want them.

This is one factor to consider when you write subject lines and the actual email content that users see.

Check out the recording of our webinar on driving email subscriber engagement.

7. Getting blocklisted

Illustration of an email getting checked against a blacklist.

Blocklists do exactly what they sound like. They amass lists of email sender IP addresses that have been flagged for spam or other malicious behavior, and then recommend they be blocked. Learn more about how blocklists work.

Internet service providers subscribe to these services and rely on them to help filter spam from their networks. To be clear, blocklists are real and legitimate companies. And if you get put on their lists, your email deliverability will suffer.

Some of the most well-known blocklists are Mail Abuse Prevention System (MAPS), Spamhaus, MXToolBox, Barracuda Reputation BlockList, Spamcop, URIBL, Google Transparency Report, SpamAssassin, and PhishTank.

How to avoid getting blocklisted

Make smart decisions. In a nutshell, that’s it. Why? Because spammers don’t.

What does being smart look like in regards to email deliverability? Here are a few best practices:

  • Never buy email lists: These are often filled with abandoned or made-up email addresses and spam traps, a surefire way to skyrocket your hard bounce rate and send the overlords after you. This may also go against your ESP’s terms of service, resulting in a ban from their platform.
  • Don’t send crappy emails: Engagement matters, and if you can keep your engagement strong, you aren’t likely to fall into the hands of the blocklisters.
  • Segment your subscribers: Don’t batch and blast your entire list all at once, and you’ll get better engagement.
  • Send consistently: Sending twice-weekly emails for months, and then sending ten of them all at once, is bad marketing. As such, it’s also a huge warning sign that you’re a spammer.
  • Use authentication protocols: Work to implement SPF, DMARC, DKIM, and BIMI.
  • Use Email on Acid: Run Email Deliverability checks before you hit send.

We check for your domain with four major blocklist companies: Spamhaus, PhishTank, Google, and URIBL. If your IP shows up, we’ll alert you and point you in the right direction to get yourself removed. Beyond that, we look up blocklists from at least 72 other services.

If you want optimal email deliverability, you should follow the best practices of email marketing and, when something goes wrong, deal with it quickly.

What to do if you are blocklisted

Fortunately, this is not a permanent stain on your record. You can request removal from some blocklists by contacting the company and following their process for verifying yourself as a legitimate sender.

Also, consider adding an email preference center and informing your readers about it. There, they can choose the types of emails they want and their preferred frequency

Do subject lines affect email deliverability?

Email Subject Lines Best Practices

This used to matter more than it does now. Ten years ago, spam filters would look for certain words, phrases, and symbols often used by spammers, such as exclamation points, all CAPS, and ‘free’, amongst other terms.

But other technological methods now do the bulk of the heavy lifting when it comes to suppressing spam. So worrying about using ‘flagged’ terms in your subject lines is less important than it used to be.

That said, subject lines still matter because they affect user engagement. Low engagement can result in reduced inbox placement. Using tactics common to spammers in your subject lines, therefore, is not advised. If those methods lead to low open rates and poor engagement, and if your emails end up in the junk folder, your engagement will suffer, and that very much does affect your sender reputation and thus your deliverability.

So, don’t be paranoid about all the words some blog says you shouldn’t put in a subject line. At the same time, don’t send your subject lines to a Halloween party dressed as spam either.

Be relevant to your readers and keep your engagement strong.

What are spam traps?

The ISPs really don’t want spam traveling across their networks. As you’ll see in a moment with some spam statistics, what they’re doing is actually working pretty well.

Spam traps are one tool they use to fight spam, especially fraudulent spam. Sometimes spam traps are referred to as “honeypots.” There are three types of spam traps: recycled, pristine, and typos. Here’s a brief look at each.

Recycled spam traps

What happens to old, inactive email addresses? If you abandon an email, such as from a previous workplace or college, that email address still exists on various lists — ones you signed up for when you were using it.

An ISP can turn that into a spam trap. They know it’s no longer a legitimate email. Anyone who continues sending emails to it either bought it on a list, obtained it through some other unscrupulous method, or just isn’t paying attention to their email list data.

If you continue sending emails to a recycled spam trap, this could land you on a blocklist or cause your email to land in the junk folder.

Pristine spam traps

Remember, spammers are kinda lazy. So, they create bots to scrub websites for email addresses so they don’t have to do it by hand because that would be too much work.

Companies know this, so they’ll sometimes bury email addresses in the code of their website that will appear real to a bot. It’s not a real email address and isn’t visible to anyone just looking at the website. But the bot will add it to the billions of other email addresses on their list.

Even one email sent to that address is automatically categorized as spam because no one else even knows the address exists. Avoiding pristine traps is easy, as long as you don’t purchase any email lists.

Typo spam traps

example of a typo spam trap
Example of typo spam trat

These work similarly to the others, but just use slight misspellings — usually in the domain. So it might say gnail, instead of gmail. Or amazone, instead of amazon.

A spokesperson from Spamhaus explained that these typos often happen when an email address is given at the point-of-sale and incorrectly entered through manual methods. However, these emails then end up on purchased lists, which makes them spam traps.

The risks of spam traps

The risks of spam traps should be obvious. These aren’t real email addresses. No real person is using them. If you’re sending emails to these, the only people who might find out about it are the blocklist companies and the mailbox providers. And their response will be to flag your domain or IP as spam and filter your mail.

How do spam traps end up on your email list?

As already mentioned, this is THE biggest reason to never purchase an email list. Where did that company selling that list get all those email addresses? It’s very probable, if not guaranteed, that some of the emails on the list you purchase will be spam traps. And mailing to them even once puts you in the crosshairs of the spam assassins. And they will not be kind.

The other way spam traps end up on your list is because they were already there. Again, someone could abandon an email address they used to honestly opt-in to your list — like when they moved from Hotmail to Gmail.

How to avoid spam traps: The double opt-in

Using a double-opt-in approach to email list building is a surefire way to prevent bots from signing up for your email list. The confirmation email you send to those bots will never be read, and this will keep fake emails from showing up on your list.

Next, you need to make it a habit to regularly purge your lists. Your internal stats should dictate the specific amount of time, but If you have email addresses that haven’t opened a single message in over a year, it’s pretty likely that they’ve been abandoned or no longer exist. Purge those emails, and you remove the chances of them turning into spam traps.

Third, do whatever you can to avoid being labeled as spam. Include an unsubscribe button in every email and make sure it’s easy to find. Pay particularly close attention to your welcome series and immediately root out any bounces. Those are generally the first email a subscriber is sent, and so it can serve as your first line of prevention.

These simple methods will protect your sender reputation and keep you off the blocklists.

A brief history of email spam

Gary Thurek - Father of Spam

How big a problem is spam? And, do we even agree on what ‘spam’ really is? This matters, because one person’s spam is another person’s helpful marketing email that solves a problem in their life. We covered the history of spam in detail, but here’s a brief synopsis:

In 1978, what is often called the first spam email was sent. It was from a man named Gary Thurek, who sent an unsolicited marketing message to hundreds of users on what was called the ARPANET system. His email pitched a product from the Digital Equipment Corporation. Thurek says that email earned $13 million in sales for his company. Not a bad day’s work.

However, if being unsolicited is the definition of spam, Thurek’s email qualifies. Of course, unsolicited email wasn’t called “spam” at the time.

There’s some disagreement about where the term ‘spam’ came from as a way to describe unwanted and annoying emails.

However, many point to a Monty Python sketch where customers at a diner get offered Spam with every meal choice on the menu, and then get harassed by a bunch of people chanting about Spam, which they have stated several times they don’t want.

In 1998, two decades after Thurek’s unsolicited email, spam had become so prolific that it merited a listing in the New Oxford Dictionary.

In 2003, President Bush signed the CAN-SPAM Act, the first major attempt by a government to set standards for commercial emails.

Email spam statistics

Reliable spam stats are actually tough to come by, and this is why the definition of spam matters.

One site claims that 85% of all emails sent in 2020 were spam. But another site, for the same year, says it’s just 50%. How can that be? Maybe it’s safe to say there’s too much spam to count.

And that’s not the only difference. One says 43% of phishing scams targeted Microsoft accounts, but the other site says the most commonly targeted are online stores. One says most spam comes from within the US, and the other blames Russia.

So, you have to take some of these stats with a grain of salt, because not only is spam hard to define, it can be hard to measure.

Here are some statistics about email spam you might find helpful:

  • Fake invoice spam emails doubled in 2020.
  • Most spam emails — again depending on definition — are not scams or fraud, just unwanted.
  • Porn and dating site spam constitutes 32% of spam messages — and these are where malware is most likely to be found.
  • For every 12.5 million spam emails, spammers get one reply, a 0.000008% reply rate.
  • Spam costs real businesses $20.5 billion per year in lost productivity and technical expenses.

The best spam statistic, however, is the one that suggests the attempts to fight it are actually working.

In six months — from June of 2020 to January of 2021 — the number of spam emails sent per day dropped from 316 billion to 122 billion.

Yes, 122 billion is still a lot. But it’s about a third of what it used to be, and that’s just in six months. So while all these blocklists and deliverability challenges are real, their reason for existing is a good one, and their methods appear to be delivering. No pun intended.

Anti-spam law basics

Spam Laws

There are four major anti-spam laws to know about, and many more in countries around the world.

CAN-SPAM – Controlling the Assault of Non-Solicited Pornography And Marketing

This is a US law signed by President Bush in 2003 which distinguishes commercial emails from transactional and relational emails. Among other things, it requires a visible and fully functional unsubscribe option, a real physical address for the sender, and accurate ‘from’ information.

CASL – Canada Anti-Spam Legislation

Canada’s law goes much further than the US one. It requires some type of positive action to be taken by the user before a company can collect their email information. You’re not allowed to presume consent by pre-checking a box.

GDPR – General Data Protection Regulation

This well-known European Union law requires affirmative and explicit consent to collect a person’s information and includes guidelines for how that can be used and stored. It also simplified what had been a hodgepodge of regulations across various EU nations. Subscribers also have the “right to be forgotten.” At their request, you must purge them from your database.

CCPA – California Consumer Privacy Act

This recent law allows any consumer in the state to demand to see what a company knows about them, and who else their data has been shared with or sold to. This, like GDPR, also includes the right to be forgotten.

See more spam laws from other nations

Email list hygiene

toothbrush and bubbles clean email list

Cleaning your list is good for business. By removing inactive emails regularly, you’ll increase your open rates and click rates, and maximize your email deliverability.

Suppose you have a list of 100,000, but 20,000 of those addresses are inactive. If you’re getting a 10% open rate, that’s lower than it really is. Purge the 20,000 dead emails. Now, with just 80,000, that same 10% open rate becomes a 12.5% open rate, and you didn’t change a thing about your actual marketing. You just removed all the abandoned emails.

Cleaning your lists also strengthens your sender reputation and reduces spam complaints, because you’re keeping the people who want to be there, and removing the ones who don’t.

See best practices on how to clean your email lists.

Implement these four tips for a healthy list:

  1. Use welcome emails. This is the single best way to engage a new subscriber and solidify your status in their mind as someone they want to hear from. Don’t wait weeks — offer a warm welcome as soon as possible after you obtain the address.
  2. Make the unsubscribe link easy to find. This reduces spam complaints to near zero.
  3. Send emails consistently and at the cadence you promised. Subscribers will appreciate the predictability but will lose their trust in your brand if your “weekly newsletter” becomes a daily occurrence.
  4. Re-engage people who haven’t opened anything in months. Give them a chance to stay subscribed. Remove any names who don’t respond.

Email deliverability best practices

Finally, here’s a summary of the major dos and don’ts we’ve made in this article:

  • DO include an unsubscribe link – it’s the law.
  • DON’T ever buy an email list.
  • DO practice consistent list hygiene.
  • DON’T skip email authentication protocols like SPF, DKIM, DMARC, BIMI.
  • DO consider using double opt-in for list signups.
  • DON’T email your lists infrequently.
  • DO choose your sender name and subject lines carefully to avoid spam complaints and maintain higher engagement.
  • DON’T batch and blast your entire email list.

Deliverability is job #1 for email marketing. If it’s not delivered, then it doesn’t exist.

Email on Acid includes deliverability checks as part of our email readiness platform. In addition to making sure your emails look perfect on over 90 devices and platforms, we also look you up on the major blocklists. If your IP shows up, we’ll give you a jump-start at removing it.

Improve Deliverability to Hit More Inboxes!

Nothing ruins a polished email’s ROI potential like a trip to the spam folder. Run a Spam Test right within your Campaign Precheck workflow so you can land in more inboxes and increase email ROI. With Email on Acid, you can check your email against 23 of the most popular spam filters and your domain against the most popular blocklists before you hit “send”. Sign up for a free trial and try it out today.

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Author: The Email on Acid Team

The Email on Acid content team is made up of digital marketers, content creators, and straight-up email geeks.

Connect with us on LinkedIn, follow us on Facebook, and tweet at @EmailonAcid on Twitter for more sweet stuff and great convos on email marketing.

Author: The Email on Acid Team

The Email on Acid content team is made up of digital marketers, content creators, and straight-up email geeks.

Connect with us on LinkedIn, follow us on Facebook, and tweet at @EmailonAcid on Twitter for more sweet stuff and great convos on email marketing.

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