About Spam Filters

Spam filters use a lot of different criteria to judge incoming email. Each factor is weighed and added up to assign a spam score, which determines if a campaign will pass through the filter. Passing scores vary depending on the server, so a campaign might pass through some filters but not others.

Even compliant senders with permission-based lists can get flagged by aggressive spam filters. The best way to avoid these false positives is to better understand how spam filters work.

In this article, you'll learn more about spam filters and how to determine if you have a problem with your content.

What Spam Filters Look For

Because different spam filters can function slightly differently, it can be difficult to nail down the exact criteria for judging spam. However, there are some basic characteristics of spam that tend to hold true. Below are some of the common things spam filters look for and how to avoid them.

Subject Lines

Effective subject lines are a critical component of effective email marketing. Subject lines that are irrelevant or contain a lot of capital letters and excessive punctuation tend to trigger spam filters. Avoid words like "free," "act now," "limited time," or "click now."

Campaign Metadata

Spam filters want to know that you are acquainted with the person receiving the email. Spam filters are more likely to flag your email if your email is addressed to your recipient's email address and not their name. We recommend you use merge tags to personalize the To: field on the Setup step of the Campaign Builder.

Spam filters will also look to see if you're already on your recipient's contact list. Spam filters usually judge senders that use anonymous and free email addresses for their "from" email, such as Gmail, more harshly than verified domains.

Ask your subscribers to add you to their address book or set up an email address using a private domain name. Campaigns sent through MailChimp’s servers are authenticated for you with every campaign you send, but you can also set up your own authentication.

Your IP Address

Some spam filters will flag your campaigns if anyone with the same IP address has sent spam. When you send campaigns through MailChimp, your content is delivered over our servers. That means if one person decides to send spam through MailChimp, it could affect deliverability for our other users.

Our Compliance Team takes spam and abuse complaints very seriously. We work vigilantly on our end to keep our sending reputation intact, so it's important that you do your part to abide by our Terms of Use.

Content and Format

Your campaign content should be clear, clean, and balanced. All of your subscribers should all have opted-in to your list and know who you are. Any kind of blurb or introduction to your company could look like an advertisement and could trigger spam filters.

Avoid financially-motivated content, such as offering a chance to win money or reduce debt, and keep a balance of images and text in your content. Be sure all your links are valid and avoid link shorteners.

Generally avoid all caps, crazy colors, and extra exclamation points anywhere in your content. Spam filters will also check to see if a plain-text version of the campaign is available.


Sloppy code, extra tags, and code pulled in from Microsoft Word can trigger spam filters.

We recommend you use one of our templates or work with a designer.

The Big Picture

Spam is a real issue and should be taken seriously by everyone in the email community. For the most part, avoiding spam filters is about staying compliant and understanding how your campaign looks in its entirety. Spam filters use sophisticated algorithms to analyze a lot of email with a long list of criteria to consider.
The big takeaway is that if something about your email triggers a spam filter, it will likely take a closer look but generally, your campaign would need to have multiple triggers to get filtered as spam. Always stay in compliance, test your campaigns, and take advantage our Inbox Inspector tool.

How to Determine if You Have a Problem

It's not always obvious if you have a spam complaint problem, but the stats in your campaign reports are a good place to start. Look for Abuse Reports on the Overview page of a campaign report. If it's more than zero, click the Activity drop-down menu and select Complaints to get more information.

Next, look at your open rate. If you notice that your open rate has dropped significantly over time, it could be due to an increase 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, look at your hard bounces and SMTP replies. If the SMTP reply mentions aborted processes or a permanent error, that could indicate a spam filter issue.

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