Here is a tip for training your SpamAssassin bayesian database
Make sure you decrypt the body of the email before sending it to SpamAssassin to learn the tokens.
When I upgraded to the latest version of Maia Mailguard, I forgot to add the decryption key to /etc/maia.conf. This caused the encrypted data to be sent to SpamAssassin. This was causing SpamAssassin to get a lot more false positives.