Have you tried disabling any anti virus scanning of your mail?

Matt, Tried disabling the anti virus email scanning. Unfortunately that did not help. Any other ideas?

can you open the attachments in your "sent" folder?

No. Attachments in the sent folder have the same flaw.

Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

In safe mode, can you send and open from the sent folder?

In the safe mode - I could send from the sent folder...if you meant...could I open the attachments that were problematic -- no -- they remained un-openable. The faulty attachments are very small...e.g. 17 bytes...when they normally are around 450 KB. So something happens to them when the Thunderbird filter tries to forward the original incoming email.

Thanks for your efforts, BTW.

No, I was asking you to send another mail (To yourself if necessary) and were those attachments an issue.

Sending emails to myself...or others manually have never been a problem.

The only issue is when the Thunderbird filter recognizes an appropriate incoming email and forwards it to other select recipients. That has worked for several years. Over the past week or so, however, the attachments that are forwarded are no longer functioning. The correctly sized and formatted incoming attachment becomes a tiny -- non-functioning file....which neither the recipients nor I can open.

Check your setting under Tools -> Options -> Composition -> General -> Forward messages. Do you forward your messages inline? Have you checked the option to add the extension to the file name?

Onno,

Yes...It is set to forward messages inline...and adding the extension to the file name is checked.