What are spam filters?

Before an email ever reaches its intended recipient, it first must pass through one or more filters – software algorithms designed to identify and stop malicious or unwanted mail from reaching the inbox.

In the past, email filters were primarily intended to block spam, phishing, and other types of unwanted email and applied largely the same filter criteria across all mailboxes. In contrast, today's filters are incredibly sophisticated and adaptive to each mailbox, message, and sender. Not only do email filters block the bad stuff, today's filters also prioritize, classify, and organize emails to make end users more productive.

In short, reaching the inbox can be no small task. The best email teams test their emails against these filters to find deliverability problems before sending.

How does Litmus help?

With Litmus Spam Testing, you can test your emails against all of the major spam filters and inbox providers. We'll provide you with direct feedback on what each filter did with your email and what you can do to improve your chances of reaching the inbox.

Which spam filters are available in Litmus Spam Testing?

There are currently five spam filter tests included in Litmus Spam Testing.

Microsoft Exchange Online Protection

Microsoft Exchange Online Protection (EOP) protects all 60 million Microsoft Office 365 cloud-hosted mailboxes, used for consumers and business customers alike. It uses the latest technology in spam detection to evaluate and score emails.

Your email receives two scores: a Spam Confidence Levels (SCL) and a Bulk Complain Level (BCL)

SCL scores are between -1 and 9. An SCL of 5 or higher is considered spam. 

BCL score are between 0 and 9. The higher the score, the more likely Microsoft believes you are going to generate subscriber complaints. Microsoft uses internal and external data sources to determine this score.

To aid in troubleshooting, Microsoft includes several anti-spam headers into each email after scanning. To find out what these headers mean (and troubleshoot your email) check out Anti-spam message headers.

Note: Litmus uses the default settings in Microsoft EOP to determine the scores of your email.

Outlook Desktop

Outlook comes with a built-in junk email filter that learns over time what you think is spam. While this is great for individual users, it's not consistent nor reliable for use across thousands of tests on our Litmus servers. To help you reliably test your messages sent to Outlook, we've added in hundreds of spam rules that have been published by Microsoft. 

This filter has various sensitivity settings so you can adjust your level of junk email protection. We have chosen 'High' as the default for all of your Litmus tests. The Microsoft Outlook filter scores from 0-10 on the high sensitivity rating, with 0 being the highest (passing) and 10 being the lowest (failing). Outlook rates an email with a 6.0 or higher as a failure. Anything lower than 6.0 is considered a passing score with the high sensitivity rating.


SpamAssassin is a popular open source spam filter which can be deployed on its own, but is more commonly found as a component in many commercial email filters. SpamAssassin analyzes emails using text analysis, bayesian filtering, DNS blocklists, and collaborative filtering databases. Your SpamAssassin test will return an overall score, plus a supporting list of issues found along with their individual scores. 

The lower your score, the less likely your email will be flagged for common spam filtering rules and heuristics. Generally, a score of 5.0 or lower is considered not spam. 

If your email fails a SpamAssassin test, we’ll provide actionable feedback to fix the issue. Here’s a full list of Spam Assassin tests we perform.

Note: Litmus tests against SpamAssassin version 3.3 with default settings.

Symantec Messaging Security (formerly MessageLabs)

Symantec Messaging Security is a leading business-grade secure email gateway, used by organizations around the world to protect corporate mailboxes from spam, phishing, malware, and other abuse. Symantec operates similarly to other spam filters by analyzing email messages based on a wide range of factors and assigning a score to indicate if the email is likely to be spam.

For help with troubleshooting, review the Symantec help documentation covering common issues and how to contact Symantec for further investigation.

Note: Symantec also provides a helpful tool to check your sending IP Reputation.


Barracuda Essentials for Email Security is another business-grade secure email gateway used by thousands of organizations globally to filter inbound email.

Barracuda's filters analyze each inbound email, assigning a score between 0 and 10 based on how likely it is to be unwanted mail, such as spam or phishing. As a rule of thumb, you should aim for a score below 5.0. The higher the score your email receives, the more likely it is that Barracuda will block or quarantine the email.

While Barracuda test results won't always reveal specifics about why your message was classified as spam. If you’re caught in Barracuda’s spam filter, these resources may be helpful:

Did this answer your question? Thanks for the feedback There was a problem submitting your feedback. Please try again later.

Still need help? Contact Us Contact Us