if you are sure nothing was changed in Thunderbird and neither in any security software, it may be best to assume the error message is right - it’s a temporary server issue. A search for the error message will point you to articles related to Exchange.

What you could do, is change your configured SMTP server to use smtp-mail.outlook.com in the Account Settings (left pane, bottom) and leave everything else as is. Personally I would add it as a second SMTP server and leave the other one’s settings untouched, just in case and to be sure you don’t mess things up. After that, you need to select the other SMTP server at the bottom right when highlighting the email address on the left, or use the button below that drop-down to add an identity so you’d be able to choose identities (and hence servers) during message composition at any time, e.g. to test whether the issue is resolved.

Does this help?

I retried another outgoing server that my provider's tech support tried during my conversation with him earlier. It did not work at that time but works fine now. Why now and not earlier: who knows??

Your suggested server did not work. But I used your advice in adding this other outgoing server. Thanks for your help and advice.