SpamAssassin Rule: MIME_BASE64_TEXT
Standard description: Message text disguised using base64 encoding
The message contains text that has been encoded using Base64 content transfer encoding but does not use a character set known to require it. This does not apply to text in the UTF-8 or big5 character sets.
This technique is assumed to be used by spammers as a form of obfuscation, presumably to bypass filters that are not MIME-aware.
The default scores for this rule can be found in the online list of tests.
For details on Base64 encoding see RFC 2045 sec 6.8
This rule is known to trigger false positives in some circumstances. See (buglist)