disable email scanning in your anti virus and try again.

Then create an exception for your Thunderbird profile in your anti virus and see if that helps.

Dear Matt,

Thank you for your answer!

I disabled the Antivirus (AVG) for e-mail checking, composed a similar mail, sent it and it worked fine. However I re-enabled AVG it and try to re-send and it worked fine again ?! Coincidence? I don't know but I hope it stays fixed! Anyway, I noticed that the Antivirus was already not checked to scan for outbound e-mails (SMTP)

I hope the problem stays fixed however I will describe the problem below a little better just in case...

This is a problem I have most of the time when I try to send e-mails with size larger than a few hundred KBs despite the kind of the attachment. In the end, I just press "cancel" at the error message box (for copying to sent folder - or drafts folder) instead of retry and then obviously I don't have the e-mail in the sent (or drafts) folder. But there are cases that after many retries it works.

Creating the exception for the Thunderbird profile folder is about stopping the anti virus from scanning the file the sent mail is to be added to. (send folder is a file called sent).

Another possible issue is the scanning of the TMP file that is created when the mail is sent. Part of assembling the email from transmission results in an NSmail.tmp/msmail.htm being created in the temp folder of Windows. If the anti virus or another process interferes here that unexpected results could occur.

One such example occurred with Avast. See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#Slowness_and_errors_with_nsemail.HTML

It might be worth noting that AVG has been purchased by avast. So under the hood you may well have more avast than AVG now. See https://blog.avast.com/avast-and-avg-become-one