by 'hang', are you seeing an error message, or does TB just quit?

The send dialog does not close - the progress bar reaches 100%, but the dialog doesn't disappear until I press cancel. The email is actually sent by the server (it's in my server's sent folder) but a copy isn't saved in TBird (account set for POP with Oauth2).

Okay, I'm at a loss here, but I wonder if there is a timing issue? It may be worth a try.

  Tap on tools > options > Advanced > General > Config Editor.
  Visit the search file section and put mailnews.tcptimeout > Click Enter.
  Look for mailnews.tcptimeout and right-click on it > modify.
  Here change the value by adding the larger value. Remember to save them.

So place a copy in would by necessity be a local folder, either in the account or Local folders. POP is not in any way linked to the server for sent mails.

You say you have an exclusion in your anti virus for your profile, but have you prevented email scanning?

I have some instructions here about creating send logs.

Please try sending a small test mail (the log shows the entire contents) and post the log. Perhaps that will contain something enlightening.

Thanks for the good advice David and Matt. Progress.

First, the immediate problem seems to be TB cannot access the temp email file created during sending. The following error occurred after the SMTP connection closed.

NotAllowedError: Could not open the file at C:\Users\username\AppData\Local\Temp\nscopy-3.tmp for writing

I checked permissions, and the file was not read only, and running TB as administrator made no difference. Adding C:\Users\username\AppData\Local\Temp\ to the exclusion list for AV seems to have fixed the problem. Is this a normal requirement (I'd rather not add unnecessary exclusions, of course)?

Second, the log reported a different SMTP error even when everything worked as expected (sending and saving a copy). Here's the log snippet from the end of the sending steps. I don't know if this is normal or relevant in some circuitous fashion, but figured was worth sharing. Note that I did increase the smtp timeout setting to 500 from 100, as suggested by David.

SmtpClient.jsm:412:17 mailnews.smtp: Message sent successfully. SmtpClient.jsm:1246:21 mailnews.smtp: C: QUIT SmtpClient.jsm:578:19 mailnews.smtp: error { target: TCPSocket, isTrusted: true, name: "NetworkError", message: "Network", errorCode: 2152398868, srcElement: TCPSocket, currentTarget: TCPSocket, eventPhase: 2, bubbles: false, cancelable: false, … } SmtpClient.jsm:433:17 mailnews.send: Sending failed; The message could not be sent because the connection to Outgoing server (SMTP) smtp.office365.com timed out. Try again., exitCode=2153066800, originalMsgURI= MessageSend.jsm:330:27 mailnews.smtp: Closing connection... 2 SmtpClient.jsm:152:17 mailnews.smtp: Socket closed. SmtpClient.jsm:486:17 mailnews.smtp: S: 221 2.0.0 Service closing transmission channel

Inability to access the temp file (which is the entire body of the email) has a rather glorious past with some antivirus products. I suggest you approach the folk providing the anti virus and ask what to do to make it do it's job without being so slow about it.

Otherwise I suggest you manually clean out your temp folder (windows collects junk there.)

That will generally make accessing file faster. The fact the file name has a number in it says the problem is out of control. There are already orphaned files in the folder Thunderbird has abandoned. You may be able to limit the exclusion to just the file C:\Users\username\AppData\Local\Temp\nscopy.tmp once you have cleaned up.

The timeout is not really relevant given accessing the file is the cause of the send just hanging.

Thanks - trying out a specific exclusion.

Any idea what's up with that additional SMTP error? (No effect on functionality, but should it be reported as possible bug?)

My, admittedly poor, understanding is the 2152398868 basically means a connection was opened and nothing was sent. Entirely consistent with everything else resulting from having nothing to send because the file was inaccessible.

I have had this problem for years on and off. No idea what to do. Don't understand the advice above. Could someone fix my problem for me ?

I have this problem on my iMac running the latest Catalina. (Later OS versions do not support my late 2013 model iMac purchased in early 2015 and no CURRENT iMac has a 27" monitor - I have two)

It is SLOW copying to SENT. It often FAILS when using the extension to COPY TO folder. But it is exceedingly SLOW for BOTH folders, IF it succeeds at all. I have folders for all my recipients, under specific categories, such as "Correspondance" or "Household>Finance" etc. It even appears to have difficulty saving to a LOCAL folder! My mail server is Google's Gmail imap server.