Mailchimp email delivery plummeting? Top 10 Most Common Spam Filter Triggers

By Ben@ Mailchimp: Here’s the top 10 list of spam filter criteria that MailChimp users are most guilty of. I’ve included the corresponding number of detected matches (keep in mind the system is not done scanning—it might take another week to finish):

1. BODY: HTML has a low ratio of text to image area (1,217 matches)
2. BODY: Message only has text/html MIME parts (971)
3. BODY: HTML has a low ratio of text to image area (729)
4. BODY: HTML and text parts are different (625)
5. Subject is all capitals (324)
6. BODY: HTML and text parts are different (279)
7. BODY: HTML: images with 2400-2800 bytes of words (211)
8. BODY: HTML: images with 2000-2400 bytes of words (194)
9.BODY: HTML: images with 1200-1600 bytes of words (178)
10. BODY: HTML: images with 1600-2000 bytes of words (178)

Number 5 is just idiotic. TYPING IN ALL CAPS = SCREAMING AND IS RUDE. Don’t type in all caps in your emails, please. Who does that?

Number 2 means somebody was lazy, and only included the HTML or the plain-text version of their emails, instead of both. I think that’s what it means. Spam filter rules can be cryptic sometimes (intentionally, perhaps).

But the rest of the detections on that list basically mean that the senders sent way, way too many images, and not enough readable text. Spam filters can’t read images. Spammers know that, so they often send spam that’s nothing but a big, ginormous image. And spam filters know that, so they in turn block email that they can’t read.

The battle between spam filters and spammers is brutal and never ending, and sometimes legit marketers get caught in the crossfire. Understand how both sides work, and do your best to cope.

But don’t try too hard to appease the spam filters. They don’t like that either (looks needy).

Not-So-Common Spam Filter Triggers

During our user research, we found some surprising spam filter triggers. Here are some examples:

The phrase, “extra inches” will get you a score of 3.1 by spam assassin. The phrase sounds like it came from some kind of “appendage enhancement” pharma-spam, right? Turns out it popped up 4 times in MailChimp, from relaxation & beauty spas. As in, “if your new years resolution is to shed some extra inches off your waistline, come in and…”
Dear FNAME, = “not very dear at all!” Do you merge the recipient’s FNAME into your messages? If so, don’t use the d-word. Turns out “Dear” will get you 2.7 spam points. That’s about halfway to getting your email blocked. Use something else, like “Howdy.” At MailChimp, we use “dear” in just about all our demo videos and tutorials, because it’s the easiest way to explain mail merge tags. When we say, “Dear *|FNAME|*,” people just get it. We might stop using this example. I’ve written about how salutations can waste valuable space anyway.
“Stop Further Distribution” – In your footer, when you give people that unsubscribe link, don’t try to be all official and corporate sounding. The phrase, “stop further distribution” will get you 3.1 spammy points. By the way—”distribution?” Nobody says that.
“You registered with a partner” – If the body of your email contains that phrase, chances are very good that your email list is not permission-based. This actually sets off a few red flags in MailChimp’s list setup process, (we get alerted when people enter that into their permission reminder), and I was pleasantly surprised to see that spam filters look for it too.
As you can see, your emails can get flagged as spam, even if you’re not a spammer. Your email delivery can suffer, even from an innocent mistake. If enough innocent mistakes happen, MailChimp’s overall deliverability can suffer. So we’re working on preventing that. Hopefully, you won’t be hearing from us soon.


There’s no magic formula—and spam filters don’t publish details regarding their filtering practices—but there are steps you can take to avoid landing in your subscribers’ junk folder.

1. Campaign metadata: Spam filters want to know that you’re acquainted with the person receiving the email. We recommend using merge tags to personalize the To: field of your campaign, sending through verified domains, and asking recipients to add you to their address book.

2. Your IP address: Some spam filters will flag a campaign if anyone with the same IP has sent spam in the past. When you send through MailChimp, your email is delivered through our servers, so if one person sends spam, it could affect deliverability for our other users. That’s why we work vigilantly to keep our sending reputation intact, and it’s important that all users abide by our Terms of Use.

3. Coding in your campaign: Spam filters can be triggered by sloppy code, extra tags, or code pulled in from Microsoft Word. We recommend using one of our templates or working with a designer.

4. Content and formatting: Some spam filters will flag emails based on specific content or images they contain, but there’s not an all-encompassing set of best practices to follow or things you absolutely need to avoid. But, we do have a few recommendations.

  • Design your campaign to be clear, balanced, and to promote engagement from your subscribers.
  • Make sure your subscribers have opted-in to receiving your emails.
  • Be consistent. Try not to stray too far from the content and design that your audience already associates with your brand, website, or social media channels.
  • Test, test, test! Use A/B or Multivariate Testing to learn how changes with your content affects delivery and engagement.



Reproduced for information purposes only from this Mailchimp Blog article dated Feb 4, 2009 written by Mailchimp co-founder Ben and this other one named How to Avoid Spam Filters (also by Mailchimp)

How Your Email Design Can Get You Blacklisted (Mailchimp blog)