spamassassin -tD < [message_file] returns a properly filtered message, then SpamAssassin is working appropriately. The best messages to use for this test are the samples included with the distribution, sample-spam.txt and sample-nonspam.txt.
SpamAssassin-Users (see MailingLists) is mainly about making the core SpamAssassin code work well, not on integrating it with other mail platforms. For integration questions, you should check with the folks who support your MTA filter (MIMEdefang, qmail-scanner, spampd, etc), using a forum dedicated to the mail system you're using.
Once you solve the problem, please consider adding info about it to the wiki, such as at FixingErrors.