Check SMTP Server Settings in Tbird Acc Settings, and try deleting SMTP password in TBird Settings-Privacy-Saved Passwords. Also open your webmail acc to see if the messages were in fact copied to Sent folder.

I don't know if this is related or not, but when I go to Outgoing Servers, it shows only a subset of the many SMTP servers that I have configured.

When I go to each account, it shows nothing in the dropdown. When I hit Edit SMTP server, it shows the correct information. When I hit OK, the dropdown is populated, and when I go back to Outgoing Server, that additional one is now in the list.

I have to do this for every account and this has happened more than once recently. It is as if Thunderbird is partially forgetting SMTP settings. That said, I don't know that is related as the timeout message does show the expected name of the server and not blank/null, etc.

For one of the ones that is failing, it is:

smtp.office365.com Port 587 STARTTLS

I had previously had Normal Password, but another person suggested changing to OAuth2. So I had done that previously but that did not solve the problem.

Is there a way to get more information from TB when it throws this error?

Thanks for any additional insights.

Thanks, Scott

I noticed that too at times, and think it might be a GUI issue, dropdowns are properly populated on Tbird restart. What if you change to SSL/TLS, and google different ports? Try also Tbird v96 beta 64-bit.

Does v96 beta address the GUI issue or might solve my timeout messages.

BTW, the message does send and one copy is generally still in the Sent folder (but with Office365, I've observed that some messages show up twice normally in Sent).

Thanks for your help! :)

Not entirely, but seldom happen to me (only once), and I fixed "not copied to Sent" Tbird error (despite it was in fact auto copied by Gmail while passing SMTP) by deleting and then readding the SMTP server OAuth2 password. It may be the cause of double copies in your case too, try switching Copy to Sent off in Tbird or Office.

You won't gain anything by installing TB beta, and might very well encounter new issues.

Gmail and MS/O365/outlook.com accounts automatically copy sent messages to the Sent Mail or Sent folders, so you must uncheck 'Place a copy in' in the Copies & Folders section of Account Settings.

Timeouts on sending can also be caused by antivirus scanning of outgoing mail, which should be disabled (leaving the real-time background scan in place).

I upgraded from 91.4.1 32 bit to 96 beta 3 64 bit. The problem continues to be the same.

When I look at both the Outlook sent and the web interface sent, I see 2 copies are there. So it doesn't appear to have had a problem with copying to the sent folder.

Message:

The message was sent successfully, but could not be copied to your Sent Folder. The message could not be sent because the connection to Outgoing server (SMTP) smtp.office365.com timed out. Try again? Would you like to return to the compose window.

Is there a way to debug/check a log further for any additional information?

Thank you!

Given later comments, I attempted to downgrade to 91.4.1 but it would not allow me to do so without a new profile. So I went back to the 96 beta for now.

I will turn off the "Place a copy" option to see if that solves the problem.

If this is automatic, will Thunderbird then pull the Sent item from the server back to the client? In other words, if I later search for content in the Sent items, I will still find it even without "Place a copy", correct?

Thank you!

As long as the correct sent folder is subscribed in TB, the folder will automatically collect sent messages that can be opened and searched. For MS accounts, this is usually named 'Sent', while for gmail it is 'Sent Mail'.

I tried to uncheck the Place a copy option, but no longer then see the item in the Sent folder and thus not in any searches, etc. as I can tell. Thanks

I am looking in the same "Sent" folder I have historically for the Sent mail. I did however find a "Sent Items" folder which does have the Sent item. It looks like "Sent" was my local copy and "Sent Items" the cloud based version which I have now subscribed to. Will continue to explore. Thank you!

Timeouts on sending can also be caused by antivirus scanning of outgoing mail, which should be disabled (leaving the real-time background scan in place).

Microsoft Defender for example doesn't list used now by TB MSF email files among currently supported (in Windows Group Policy Editor - Admin Templates - Windows - Defender Scan options). Does TB send and receive email files in a different format compare to storing them locally?

I am using Norton and have for many years with no prior issues. The timeout error is only in office365 mail accounts from within TB. I checked Norton logs and did not see anything related to this.

The error is also intermittent but common.

Thanks in advance.

The TB profile folder location is shown in Help/More Troubleshooting, about:profiles, and it can be added to Norton settings by following these instructions:

https://support.norton.com/sp/en/ca/home/current/solutions/v3672136

I too have this problem. The message sends ok but fails to save a copy in the Sent folder. As well as the TB message I usually get a brief system message saying the connection failed. If I hit the Save button it saves the message in the offline folder so at least it's not lost. I've consulted my domain and broadband providers but they say there is no problem they can detect, and it must be an issue with TB. The above thread seems to confirm that. I've been using TB 91.9.1, just updated to 91.1.0 on Windows 10 v21H2 (19044, 1706). Now it's failing to send messages altogether - the error message is saying the connection to the smtp server has timed out. Help!