man qmail-smtpd excerpt:
Unacceptable base64 encoded LOADER types in message. qmail-smtpd will reject every message if 5 significant characters (e.g. Mi5kb) in any of it's embedded base64 attachments is identical with one compiled into badloadertypes.cdb. Use qmail-badloader-types to derive badloadertypes.cdb from badloadertypes. The control file badloadertypes.cdb is evaluated if the environment variable $BADLOADERTYPE='M' is set and the provided character matches the first character in badloadertypes.cdb.