Even permission-based lists can fall prey to aggressive spam filters, which we estimate block 10-20% of email from subscribers' inboxes. We call these "false positives," and you can reduce your chances of getting them by understanding how spam filters work.

There's a pretty long list of criteria that spam filters consider in judging a campaign's spaminess. Filters weight each factor, and add them up to a spam score, which determines whether a campaign will pass through the filter. Passing scores are determined by individual server administrators, so unfortunately, what passes some filters doesn't pass all of them.

In this article:

What's considered "spammy"?

The list of what qualifies as "spammy" changes often, because the filters adapt based on what subscribers mark as spam in their inboxes. The content of your campaign is important, and contributes to your spam score.

Try to avoid these spammy content pitfalls:

  • Formatting: ALL CAPS, crazy colors, and extra exclamation points!!!!
  • Content: Anything about getting money, paying less money, or money-back guarantees
  • Code: Sloppy code, extra tags, code pulled in from Microsoft Word
  • Images: Too many images, or one single image and no text to balance it out

There's a lot more to your email than just the message body though. A spam filter's job is to rate how familiar or relevant you are with your recipients. It will also analyze the subject line, the recipient's name, your From: name, the sender's IP address, domain, and reputation, and even advice from other spam filters.

What do spam filters check?

Subject Line

The subject line is a critical component to judging spamminess by spam filters and human beings. Relevance is key. Subscribers should know at a glance who sent the campaign and what it's about. A subject line should be intriguing but not misleading.

Improve your score: Avoid words like free, mortgage, insurance, act now, casino, limited time, coupons, click now, and open immediately; don't use too much punctuation or special characters, including creative numeral use (e.g., H3LLO, ~*Hi*~).

To: Field

Spam filters want to know that you are acquainted with the person receiving the email. Instead of sending emails to freddie@mailchimp.com, a spam filter would rather see your email addressed to Freddie or Freddie von Chimpenheimer.

Improve your score: Use merge tags to personalize the To: field on the Setup step of the Campaign Builder.

Message Content

It's important to consider your audience when you're creating any type of content. Since your subscribers have opted-in to your list, they should already know who you are, which means you can avoid any kind of introductions about you or your company. Familiarity is key to avoiding spam filters.

Your campaign content should be clear, clean and balanced. That means it's free of "spammy" words and punctuation, the code is valid and uncluttered, and there's a good balance of images and text. Links should go directly to valid targets or via link shorteners that have not been blacklisted.

Spam filters also check to see if a plain-text version of the campaign is available.

Improve your score: To keep your campaign code clean, use MailChimp templates or work with a designer. We strongly advise against using Microsoft Word or Publisher to code your campaigns.

Your IP Address

Where your campaign comes from is as important as what's inside it. Some spam filters check your sending IP against blacklisted IP ranges. Unfortunately, that means if anyone in your IP range is spamming, your legitimate messages can be trapped in filters too.

Blacklisting is of particular concern to MailChimp, because if one of our millions of users does something spammy, it can affect the deliverability of our other users. That's why you might hear from our Compliance Team from time to time.

Improve your score: Stick with MailChimp for the most part, we're vigilant so you don't have to be. But you'll have to hold up your end of the bargain by staying in compliance (and telling your colleagues about blacklisting too).

The From: Name and Email Address

Another way for filters to check whether you're acquainted with a recipient is to verify the sender against the recipient's address book. If the filter doesn't find you in the recipient's contacts, it may push your campaign straight to the spam folder or judge your content more rigorously.

Spam filters often judge anonymous free email addresses more harshly than verified domains. If you're using a Yahoo, Gmail, or other free email address to send your campaigns, it's not guaranteed that your campaigns will get filtered, but your chances are higher.

Improve your score: Set up an email address using your company domain name. Ask your subscribers to add your email address to their address books. Include this request on your subscription confirmation screen and Welcome emails, or even place it at the top of each campaign you send.

Your Domain Name

Part of checking out your email address and sender reputation has to do with your domain name and whether it has been authenticated. Spammers use fake and spoofed domain names, so filters want to be sure yours is valid.

Improve your score: MailChimp handles authentication for you with every campaign you send, but you can also set up your own authentication if you're comfortable with that sort of thing.

Advice from Other Spam Filters

Many webmail services and ISPs use community-based reporting to block spam. If too many of your subscribers click the Report Spam button in their inboxes, that trend is noted and used to determine whether future campaigns are likely to be spam.

Improve your score: There's no way to control what your subscribers do with your email or which ISPs hold their feedback against you. But if you take care of your permission-based list, and send relevant content with engaging subject lines, your subscribers are more likely to look favorably on your campaigns and keep their fingers off the spam complaint button.

The Gray Area

Spam is a real issue, and making a great email campaign is serious business, but don't worry too much. Sophisticated algorithms spam filters use aren't black and white. They have to scan and analyze a lot of email, and they use a well-rounded combination of these criteria. So, don't be afraid to say "click here" or talk about certain offers or discounts, if that's the kind of business you do. As long as the rest of your email is in good shape, spam filters will let you through.

How do I know if I have a problem?

It's not always obvious if you have a spam complaint problem, but you can start by looking at the stats in your campaign reports. Look for Abuse Reports on the Overview page of a campaign report. If it's more than zero, click the Activity tab and select Complaints to get more information.

Next, you'll want to look at your open rate. If you notice that your open rate has dropped significantly over time, it could be due to an uptick in spam complaints. Check your rate against our industry benchmarks, to see how you're performing alongside your peers. In general, you should aim for 20-30% opens for most campaigns.

Finally, do you have a lot of hard bounces? MailChimp provides SMTP replies, which are the responses from your recipients email servers. Those replies can offer clues as to why a message bounced, including aborted processes or a permanent error, both of which sometimes suggest spam filtering.


The Inbox Inspector is commonly used to see how your campaign will look in email inboxes, but it can also tell you if any filters trapped your campaign and check your Spam Assassin score. Spam Assassin is a third-party tool that looks for spam triggers, assigns weighted values to each, and provides a score from 1-5. Anything under 3 is considered safe for most filters. To view yours, click the Spam filter check tab on the Inbox Inspector report.

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