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.
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.
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:
Next, let's break down all the things that fall underneath the big 3, to exhaustively cover every possible factor.
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:
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.
Your domain is the website your emails are coming from (@business.com). 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.
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.
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:
If you find your emails are ending up in spam, you may want to use a free blacklist monitoring tool (like https://mxtoolbox.com/) 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 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.
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.
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.
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.
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.
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.
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.
In addition to watching out for content quality and trigger words/topics, you should also avoid the following:
The quality of your email list essentially tells inbox providers the quality of your business. Here's what to watch out for:
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.
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!).
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.
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.
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.
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.
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.
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 😅
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:
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.