About Spam Filters

Spam filters use a lot of different criteria to assess incoming email. After looking at each factor, spam filters assign a spam score. This score determines if an email will pass through the filter. Passing scores vary depending on the server, so an email could pass through some filters but not others.

Even compliant senders with permission-based audiences 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

Not all spam filters function the same way, so 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. Here are some common things spam filters look for, and how to avoid them.

Campaign Metadata

Spam filters are more likely to flag your email if it’s addressed to your recipient's email address and not their name. We recommend you use merge tags to personalize the To: field of your email. Spam filters will also look to see if you're already on your recipient's contact list.

Anonymous and free email domains, such as Gmail, are often filtered more aggressively than verified email domains. However, even private domains need time to develop a good sending reputation. It can be risky to send a big email campaign from a brand new domain before your reputation is established.

We recommend that you ask your subscribers to add you to their address book, or set up your email address using a private domain name. Email campaigns sent through Mailchimp's servers are authenticated for you, but you can also set up your own authentication.

Some spam filters might respond to your email with a Captcha test to verify that you’re a human. Make sure your Reply To: address is accurate, and check the automated replies for each email you send.

Your IP Address

Some spam filters will flag your emails if anyone with the same IP address has sent spam. When you send email 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

Email content should be clear, clean, and balanced. Private domains and some Internet Service Providers (ISPs) may have more aggressive spam filters that flag emails based on specific content or images. There aren't any hard and fast rules about how to format, but we recommend using A/B Testing and Multivariate Campaigns to test how changes to your content affect delivery to private domains and ISPs.

Here are a few things to keep in mind about content and format.

  • All of your subscribers should have opted-in to your audience and be familiar with your brand.
  • Design your campaign content with your specific audience in mind, but keep visual branding consistent. Ideally, the design of your Mailchimp campaigns should match content found on your social profiles and website.
  • Test all links before you send your email, and avoid link shorteners.


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 of our Inbox Preview 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.
Finally, look at your 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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