I'm running Thunderbird version I've set up a message filter
that automatically forwards certain e-mails as they hit my Inbox to
certain recipients. That works fine, except that I receive a mail
delivery failure notification from my SMTP gateway:

This message has been rejected because it has
a potentially executable attachment "Mail delivery failed:
returning message to sender.eml"
This form of attachment has been used by
recent viruses or other malware.
If you meant to send this file then please
package it up as a zip file and resend it.

Is there a way to stop Thunderbird from creating this attachment? The
..eml file contains a copy of the e-mail, which is unnecessary since the
e-mail is already being forwarded in-line.