Thunderbird: Forwarding Message results in ‘Mail delivery failed’ error

Have you ever forwarded a message (for example to a Gmail account) in Mozilla Thunderbird,  and gotten one of these pesky message?

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  email@example.com
    This message has been rejected because it has
    a potentially executable attachment "Hey Buddy.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.

I have. This error baffled me because I never had problems forwarding messages before and just wrote it off as a “quirk” in Thunderbird. However, after doing some research (and carefully reading the error message), I found out that Thunderbird is forwarding the email thread as an .eml attachment.

To resolve this issue, you must change an option to forward messages Inline, rather than an Attachment.  To change this option, navigate to:

Tools –> Options and click on the Composition icon

Next to the Forward Messages option, click on the drop-down box and choose the option Inline. Click OK.

Now try forwarding a message… This one won’t bounce back.