How To Report A Bug In SpamAssassin
First, make sure it really is a bug; it might make sense to ask on the "users" mailing list first (see MailingLists).
Report bugs on our bugzilla at http://issues.apache.org/SpamAssassin/ .
If you are reporting an error in how particular mails are processed, or the behaviour of specific rules, please attach sample messages to the bug report. Do not cut and paste them into the 'Description' field. This causes them to be corrupted, and there's a good chance that certain key elements of the formatting will be discarded.
Attach messages as RFC-822 format -- with the "Received:", "From:", "To:", "Subject:", etc. headers, intact, then a blank line, then the message body in raw MIME structure form. UNIX "mbox" format (as used by Eudora, Mozilla Firebird, Evolution, KMail etc.) is fine, too.
Include as much of the header data, intact, as you can. Do not go overboard censoring "irrelevant" aspects of the mail, such as addresses or Received header lines, since there's a chance you might remove some key feature causing the message to not reproduce the condition anymore.
If you can, test with "spamassassin -t < message.txt" and ensure that reproduces the problem.
This article is full of more good advice on effective bug reporting.