Content filtering is a technology that scans email, including the header, footer, code, HTML markup, images, text color, timestamp, URLs, subject line, text-to-image ratio, language, attachments, and more.
- Some content filters scan all parts of the message.
- Other content filters look only at the structure of an email.
- Still others simply parse URLs out of the message and compare them against blocklists.
Fingerprinting is a technology that helps spam filters make decisions about email. Fingerprints are hashes or checksums of email message content. These hashes, which are about 64 bytes, are many times smaller than the email content they are generated from, making them easier to store. Fingerprints are often placed in an email header by a spam filter company, which its members use to help identify spam.