Make no mistake about it—Mozilla’s Thunderbird is a great email client, and all my friends on PC’s are big fans (clients for the most part tend to be on locked down Windows boxes where Outlook is the IT choice of no-choice). But what’s up with Thunderbird v2.x sending forwarded messages as .eml attachments by default? What the fudge Mozilla?

I got this message today from a client whose email as well as website I host:

Hey mate, is there any way you can see if you can stop the mail server from blocking .eml attachments, as every message I forward is getting bounced back to me?

And here is the error message he was receiving:

This message has been rejected because it has 
a potentially executable attachment "Message Subject here.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.

After briefly considering using Exim Editor to do what the client was requesting for, I don’t know, maybe two seconds, common sense quickly prevailed and, a speedy 0.21s search query of Teh Google later, was revealed to be less than prevailing over at Mozilla. As of version 2.0, Thunderbird is configured to forward messages as attachments—by default!

The solution to this “known issue” is rather simple. Don’t complain to your host or ISP, but rather change Thunderbird’s preferences to forward messages inline:

How to forward messages inline in Thunderbird 2.0

Windows:
Tools > Options > Composition > General tab, switch “As Attachment” to “Inline”.
Mac:
Preferences > Composition > General tab, switch “Forward messages” to “Inline”.