Email Deliverability: An exhaustive checklist of all factors (2024)

As co-founder of Audienceful (an email sending platform), I get to see email deliverability data from thousands of businesses on a daily basis. This has given unique insights into what works and what doesn't.

Preventing emails from falling into spam/junk is one of things people ask about the most. So my goal with this post is to offer you an exhaustive checklist for troubleshooting any potential issue.

My promise: if you do everything in this article, you will have done literally everything possible to optimize your deliverability. However a disclaimer is warranted here: nothing about email is bulletproof. Even trillion-dollar companies struggle with this.

Important stuff to know

Email is older than the web, and started as a totally open system. It used to be you could set up your own email server at home, and send anything to anybody!

By the early 2000s however, the spam/fraud problem had gotten out of hand, and email started to slowly transition into a self-regulating oligopoly of trusted inbox providers and trusted commercial senders.

As of 2024, Gmail and Outlook will basically block any mass email not coming from a server at one of these senders (called SMTP relays). These relays are largely undifferentiated and set a price floor on commercial email. This is what mostly solved the spam problem for good.

Virtually all email marketing platforms (including ours), have to send emails using these costly SMTP relays on the backend. Otherwise your emails don't get delivered. More about this in the IP section below.

Overview: The Big 3 Factors

Now that you understand how email works in 2023, let's start from first principles. Every single thing that affects deliverability falls under one of these three factors:

  1. Your Reputation
  2. Your Content
  3. Your Email List

Next, let's break down all the things that fall underneath the big 3, to exhaustively cover every possible factor.

1. Your Reputation 🏆

When troubleshooting an email deliverability problem, you should start here. Before inbox providers even see the content of your emails, they will see the following about you:

SPF, DKIM and DMARC authentication

Every email gets checked for 3 signatures that verify your identity and prevent spoofing. The first two, SPF and DKIM, should be configured already if you got green checkmarks when adding your domain to Audienceful (or any email platform). The last one, DMARC, is up to you and can be a little more tricky. We usually recommend using a free outside service for this like Postmark's DMARC tool.
*Action item: Make sure all 3 are set properly. Beginning in February of 2024, Gmail will refuse to deliver mass emails from domains that don't authenticate.

Domain reputation

Your domain is the website your emails are coming from ( Your domain acts as a point of reference for inbox providers. They keep a history of its age, what you send, what engagement you get, if people reported spam, etc. and use this to calculate a domain reputation. If you intend to build a lasting business, its highly recommended you send from your own domain to start building a reputation.

MX records and SSL certificate

Make sure your root level domain is setup to receive emails (some good options are Google Workspace, Fastmail and Zoho). Also ensure your website works when visiting the "https://" version of your domain. If you don't have an SSL certificate, it's usually a 1-click thing with your web hosting provider or you can setup a free cloudflare account.

IP reputation

This is the server that your emails are being sent from. This used to be a very important spam factor, however, is becoming less so (assuming you don't try to run your own email server). Gmail and groups like Spamhaus have had to adapt the fact that 98%+ of companies now send emails via SMTP relays (like Audienceful does) that use the same shared commercial IPs. More on this:

  • Shared vs. Dedicated IP: by default, on every email platform you'll be sharing IPs with other customers. This might sound undesirable, but its actually to your benefit. If you're not sending millions of emails, you're not big enough for inboxes to care (you're effectively an unknown). So we do not recommend Dedicated for a vast majority of customers. But if you're sending millions per month and have the time to manage it, a dedicated IP can provide the best reputation. With Audienceful you can add a dedicated IP to any growth or business plan.
  • Shared IP neighbors: when using shared IPs, the quality of those IPs will directly correlate with the quality of the platform you're using. Email is a mostly efficient market, so if you're using a dirt cheap email sender, you'll be sharing IPs with folks where price is the #1 factor (cold email spam is a numbers game). Also, some industries get worse inbox placement (eg. ecommerce), so if you're using a tool targeting ecomm, expect to see yourself in the promotions tab. Never use a platform that allows emails for cryptocurrency, gambling, supplements, adult services, etc. in their Terms of Service.

Blacklist monitoring

If you find your emails are ending up in spam, you may want to use a free blacklist monitoring tool (like to check if you've been placed on any blacklists for your Domain or sending IP. If its your domain, you can reach out to owner of the blacklist and request to be removed. If its IP, and you're on a shared IP, this will typically resolve itself within a few days/weeks as most SMTP relays have full time employees tasked with solving this. If you're on a dedicated IP you have to stay on top of it yourself.

BIMI record (optional)

BIMI is a way to get your logo to appear in people's inboxes next to your emails. On some inbox providers, all you need is another record on your domain and it works. Here's how to set it up. One big caveat however... some inbox providers like Gmail require a Verified Mark Certificate (VMC) first, which costs $1,500 per year. We list this as optional since we aren't aware of any inbox providers that use it as a deliverability signal, it's more of a nice-to-have.

2. Your Content 📑

Inbox providers don't have eyes. They also can't read images (yet). But they do speak the language of text and code contained in your emails.

Email template design & accessibility

Gmail knows that emails from real people and important transactional emails (receipts, password resets, etc) typically have simple designs—single column, just text and images, etc. These emails have the best chance of reaching the Primary tab. We recommend not trying to build emails that look like mini-websites. They're a guaranteed ticket to the Promotions tab, and rarely work well on mobile/dark mode.

Physical address and unsubscribe

If your footer doesn't contain a physical address (required by the CAN-SPAM act) or an unsubscribe link and is coming from a mass email service, this can be a red flag. In our platform we include an unsubscribe link by default on all emails.

Sending frequency and consistency

Established and reputable businesses tend to send emails on a consistent schedule (monthly, bi-weekly, weekly, etc). Also, email lists don't typically grow that fast. If one day you're sending to 10,000 people, and the next day you're sending to a 100k, that can be a signal that you may have purchased a 3rd party list.

Content quality and formatting

Always use a spell checker to prevent any broken language and avoid non-standard formatting (eg. all caps, lots of exclamation points, non-standard characters, creating words out of emojis, ASCII art, etc). These are all tactics commonly found in spam emails and will land you in junk/spam.

Spam trigger words and topics

Some of the earliest spam filters detected words/phrases and patterns. Most still do this. Avoid using super sales-y language and words associated with certain industries (eg. like crypto, NFTs, Forex, gambling, online dating, non-FDA approved pills, etc.). At Audienceful, we explicitly ban many of these industries in our terms of service to ensure it doesn't affect the rest of our customer base. Do not use any platform that does not ban these categories, as they become magnets for spammers.

Red flag tactics

In addition to watching out for content quality and trigger words/topics, you should also avoid the following:

  • No image-only emails: Inbox providers can't read images, so they get suspicious when you send an email that doesn't have much text content within it. This is a common tactic used by spammers to get around the issue with spam trigger words.
  • No link shorteners: Do not use any commercial link shortening service like These are all used by spammers to cloak links to illicit sites and are a big spam marker.
  • No attachments: This one's pretty obvious. If you've sending a lead magnet to subscribers (eg. like a PDF), never attach it directly to your email. Always link to an outside storage provider like Google Drive, Dropbox, etc. Audienceful blocks all attachments by default.
  • No outside image hosts: this typically isn't a problem if you're using a platform like ours (we host all customer images by default), but do not embed any images from a public image hosting service.

3. Your Email List 👨‍👩‍👧‍👦

The quality of your email list essentially tells inbox providers the quality of your business. Here's what to watch out for:

Opt-ins only, no cold lists

If you're following GDPR, you're already doing this, but its important you only send emails to people who have explicitly signed up to hear from you or have an existing relationship with your business. Never send mass cold emails, especially to email lists you've compiled from public sources. Otherwise you're at huge risk for being reported as spam and/or blacklisted. Our company will ban you for doing this.

Engagement or list "warmth"

If people aren't opening, clicking, or responding to your emails, this can become a problem over time. Due to iOS Mail Privacy its harder to detect these days, and it varies by industry, but if your open rate is below 15% it's a bad sign. One way to tell inbox providers your emails are important is to encourage direct replies from your readers — often in a welcome automation (side note: don't send from a 'donotreply' email address!).

Spam complaints

You should try to keep spam complaints under 0.1% on each send (1 out of a thousand people). That ratio isn't as relevant for smaller lists (eg. under a few hundred contacts), but if you go over this theshold for too long you're at risk of being banned by your email platform.

Spambot or spamtrap signups

A problem that has grown over the decades is spambots which search the web in automated fashion for signup forms where they can enter their fake addresses. To block this you should implement a spam honeypot on your signup forms and/or use reCAPTCHA. These email addresses will never engage with your emails and will bounce, and some blacklists use them as a signal.

Soft bounces

Also known as "temporary failures." These can occur because of 15+ different reasons. The big ones are typically because your recipient has an out-of-office responder, has a full inbox, had some server downtime, or the specific content of your email triggered some blocking rules at the inbox provider. Try to keep your bounces under 2%. We recommend not re-sending the same email if it has soft bounced, this can quickly turn into a red flag.

Hard bounces

These are known as "permanent failures" and more definitive. They happen for a variety of reasons, but basically mean an email address is not deliverable from a commercial sender. This is either due to a permanent block or firewall rule at the inbox, or the email does not exist. It can also result from consecutive soft bounces on earlier sends. Like all platforms, we remove hard bounced emails from your list by default.

List cleaning and age

Email lists tend to rot over time. People's domains expire or they change jobs and their email address dies. Depending on industry, a good rule of thumb is if an email address hasn't opened 1 of your last 10-15 sends (especially if they've never opened a single email from you), you should remove it from your list.

Cranky IT admins

If you're sending emails to someone with a company email address (especially a small and/or old one) that has their own email infrastructure, they can have firewall rules blocking any marketing emails. In one fun instance, we discovered a customers' IT department had blocked the entire IP range of an SMTP relay used by nearly 1,000,000 companies! Now that's a strict blacklist 😅

Troubleshooting: where to start when diagnosing a problem

Okay, so now you know every single factor that could be causing an issue. But where to start? When dealing with a customer who has an issue, we tend to go through things in the following order depending on their situation:

  • Are you sending from your own domain? If no, start by hooking that up. It's the number 1 thing you need to build a reputation if you're serious about email marketing. If yes, jump to the next bullet.
  • Are you sending for the first time? Verify your emails pass SPF, DKIM, and DMARC. You can do this by viewing the source code of the email in your inbox. In Gmail, click the three dots at the top right of the email and choose "show original." If you're not seeing a PASS next to SPF or DKIM, that's the culprit. If both say pass, then move on to evaluating content factors and your email list.
  • Have you sent emails to your list before, but not in the last 12 months? If so, you'll probably need to slowly warm up the list again on a consistent schedule to remind people and inboxes who you are.
  • Have your open rates suddenly dropped dramatically? Assuming you haven't fiddled with your DNS settings on your domain to break SPF, DKIM or DMARC, I'd start by evaluating content-related factors. Make sure the email you sent didn't contain any spam red flags, or that previous emails weren't being increasingly reported as spam by users. After this, I'd move to a free blacklist checker to see if your domain fell onto a blacklist.
  • Are certain email addresses having issues recieving your emails? Assuming its not as broad as "all Gmail addresses" (in which case Gmail might have blacklisted you — check Google postmaster tools), if you're seeing okay open rates but specific people aren't recieving your emails, it can be a problem with those recipients specifically. Make sure your emails aren't soft/hard bouncing due to their specific inbox configuration. If it's a corporate email address, often an overly-strict firewall is to blame.
  • Still having trouble? Then it's time to hit the whole factor checklist exhaustively.

That's everything.

I have passed this around within my group of felllow email nerds and believe this should cover literally everything without getting too deep. However, if you believe I've missed something, don't hesistate to reach out via email or twitter and I can add it to this doc! I will come back to update over time so hopefully it can remain the ultimate resource on this topic.

Small plug, if you're looking for a modern email platform designed for the 2020s, check out Audienceful. We're growing fast and so far our customers seem quite happy.

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