Spam filters and how they score emails

With Litmus Spam Testing, you can test your emails against major spam filters and inbox providers. You'll see direct feedback on the filter's action on your email and suggestions to improve your chances of reaching the inbox.

Why spam testing matters

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

Past email filters were generally built to block spam, phishing, and other types of undesired email. They largely applied 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 help end users get the mail they want more efficiently.

Spam filter scores available in Litmus Spam Testing

Outlook Desktop

Outlook comes with a built-in junk email filter that learns over time what mail recipients 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. Whenever the content in your campaign triggers one of these rules, we'll provide you with feedback on what can be changed to make your email look less "spammy" to Outlook.

This filter has various sensitivity settings so individual users can adjust their level of junk email protection. Litmus uses the High Sensitivity setting for in-app spam tests. The Microsoft Outlook filter scores from 0-10 on each sensitivity rating, with 0 as the top score and 10 as the worst. Outlook fails an email with a 6.0 or higher score. A filter score lower than 6.0 passes with the High Sensitivity setting.


SpamAssassin is a popular open source spam filter that 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.


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, you should aim for a score below 3.5. The higher the score your email receives, the more likely it is that Barracuda will block or quarantine the email. Please note, for some very spam-like emails, Barracuda will quarantine the email. In these cases, Litmus will report the email as considered spam and mark it with a score of 10.0 (the maximum).

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:

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 Level (SCL) and a Bulk Complaint Level (BCL).

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

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.

Webmail/ISP clients

Emails that have successfully been received in the email will show as a "passed" score. Anything that has a "failed" score will indicate that the email was not received in the inbox, which means the email could have been blocked or sent to the junk folder for that particular email client. You will also want to make sure that you are sending in your email to the complete list of seed list addresses in order to see the accurate reputation scoring for your email.

Part of the issue of testing for spam on webmail providers (such as Gmail, Yahoo! Mail,, and AOL Mail) is that a large portion of their spam scoring is based on internal metrics and algorithms -- for example, it's common for a single email to send to a single inbox on Yahoo! to be delivered successfully, but that same email may become suspicious when sent to ten or hundreds of thousands of recipients. Sometimes, a portion of recipients complain or do not engage with the email, leading the rest of the send to be delayed or marked as spam. Trends in engagement from a particular sender or IP may also produce blocks or delays, as do spam filters that "learn" about what individual users consider spam through open, click and reporting behaviors. These scenarios are difficult, if not impossible, to predict.

The webmail filters do not provide us any further information as to why an email has not been received in the inbox. You may want to check warnings produced by server-side filters (like Barracuda or SpamAssassin) and update your email with the changes those filters have suggested. You may also want to check with your Email Service Provider to see if users for a particular webmail service have marked your previous campaigns as spam in the past, as this can affect your spam performance for your email campaigns sent now.

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