man qmail-smtpd excerpt:
Unacceptable base64 encoded MIME types in message. qmail-smtpd will reject every message if the first 9 significant characters (e.g. TVqQAAMAA) of any of it's embedded base64 attachments type is identical with one compiled into badmimetypes.cdb. Use qmail-badmimetypes to derive badmimetypes.cdb from badmimetypes. The control file badmimetypes.cdb is evaluated if the environment variable $BADMIMETYPE is set.