How a spam filter works

di di Mattia Giovanni

More information about spam filters and how to determine if you have a problem with your content

Spam filters help customers work better and those who advertise properly, 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. Here are some of the common things spam filters look for and how to avoid them.

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 of your email.

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. However, even private domains will develop their own good or bad sending reputations, so it can be risky to send a big campaign from a brand new domain, before your reputation is established.

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, your content is delivered over our servers. That means if one person decides to send spam through, 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

Campaign 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 to format, but we recommend using A/B Testing and Multivariate Campaigns to test how changes to your campaign content affect delivery to private domains and ISPs.

  1. All of your subscribers should have opted-in to your list and be familiar with your brand.
  2. Design your campaign content with your specific audience in mind, but keep visual branding consistent.
  3. Be sure all your links are valid 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 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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