I've also tried all three network security options for the SMTP server. Makes no difference.

I recommend starting Windows in safe mode as the best way to rule out the effect of startup programs such as antivirus, which often block TB after an update.

Thanks for the suggestion, but I'm afraid the problem persists in safe mode.

My Thunderbird retrieves mail from three accounts, and one account has four identities. I tried systematically working through the lot. A couple suddenly sent as normal... but when I tried to repeat the experiment, they failed again. That made me wonder about a server-side problem. But then how come Gmail has no problem interacting with the server? And the server going wrong at the very moment that Thunderbird updates sounds like too much of a coincidence. None of it makes sense! It's getting late this side of the Atlantic, and I've been messing with this for hours now. I'm going to bed, and will see any further suggestions tomorrow.

What is the server name? Are you using SSL/TLS connection security? What port?

Thanks for your interest, Matt. FYI, I tried all three connection security options, with the corresponding default ports. However, it's all irrelevant now, because today, the problem has mysteriously disappeared. I really don't understand, but am leaning towards thinking that there was a problem with the server, and it was pure chance that it surfaced just after Thunderbird had updated. I did consider that possibility early on, and thought I had ruled it out by checking that Gmail *could* connect to the server.

I don’t believe the problem is with the servers. Immediately following the upgrade my lap top could not send or receive any messages however my iPad is still working on the same accounts. The problem rests firmly with the upgrade.

rrmcgregor said

I don’t believe the problem is with the servers. Immediately following the upgrade my lap top could not send or receive any messages however my iPad is still working on the same accounts. The problem rests firmly with the upgrade.

Did you post in this thread in error? I see nothing in your statement that might assist the original poster with their issue. WEre you perhaps trying to post your own topic. If so start here https://support.mozilla.org/en-US/questions/new/thunderbird

If you have updated Thunderbird and receiving mail simply stopped, try 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. If that works it is probably your security software blocking the updated version of Thunderbird. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX