SpamAssassin is a widely-used mail filter designed to identify spam by testing a number of email components:
- email headers
- subject line
- body content and formatting
- character sets
- message encoding
- sending IP
SpamAssassin runs these tests to calculate an aggregate score that can be used to determine whether or not a message is spam. The popular choice among those employing SpamAssassin is to mark anything with a score of ‘5’ or more as spam.
While it’s possible for a message with a score lower than ‘5’ to be marked as spam (because receiving mailboxes could use ‘1’ if they wanted to), it’s important to utilize the score breakdown to identify issues in your email and take corrective action.
You can view more SpamAssassin tips here: https://wiki.apache.org/spamassassin/UsingSpamAssassin