Check your network upload speed? https://www.speedtest.net/

Upload speed is 109.62 Mbps and internet service has been stable from before the problem started until now. So that's not the problem. But thanks for suggestion.

The gmail size limit for attachments is 25 MB, so if you were sending files larger than that, you must have had an add-on that uploaded to Google Drive and included a link in the message, or you used a Filelink provider add-on to do the same. If it was through an add-on, it might not be compatible with 128.

I think you've identified the problem. I see from Google that the Gmail limit has been in effect for years. With 128, I never (consciously) installed an add-on ... it just worked seamlessly. The Nov 27 (automatic) Thunderbird update from 115 to 128 "killed" whatever was doing the job. Oddly enough, I can still send large files ... they just take a lot longer to go out. So something is still happening behind the scenes .... it's just a LOT slower? The day I submitted this problem, I also changed mailnews.tcptimeout from 100 to 1000 to prevent timeouts as a temporary "solution."

Do you know any add-on for 128 that would speed up the process?

Thanks!

Timeouts are often caused by antivirus apps scanning mail. Test this by running Windows in safe mode, which bypasses startup apps like antivirus. The TB profile folder should b excluded from AV scanning anyway.

This is separate from the issue of sending attachments that exceed the size limit. Look under Settings/Composition/Attachments to see if any Filelink providers are in use.