Start Windows in safe mode with networking, then run TB. Any difference?

What is the smtp server name? It's unlikely that it works with any settings besides the ones specified by the mail provider.

Safe mode changes nothing.

Obviously, I know how to put in the correct e-mail settings (screen provided) because they worked for years until suddenly one day something broke in Thunderbird.

drdgiles said

Safe mode changes nothing. Obviously, I know how to put in the correct e-mail settings (screen provided) because they worked for years until suddenly one day something broke in Thunderbird.

No something broke, and it is most likely not in Thunderbird, it just surfaces in Thunderbird because it is the only mail client you use. When things change "suddenly" is is rarely Thunderbird and most usually security software. That is not a guarantee, but it is te normal course of events.

What operating system is it? Is your router enabled to allow outgoing connections on the relevant ports? I would check server configs, but someone conveniently removed the server names. Often with "domains I own" folks it is DNS issues that see problems occur. Do you have an anti virus that might be scanning SSL connections? Why it would not be disabled in Windows safe mode, I do not know. But I have seen software firewalls block all ports when they shut down, so not being better does not exempt a software firewall. I have seen folks on this forum have to remove and reinstall their anti virus product before it would stop blocking Thunderbird.

There are those that install edge systems to protect them from malware and just blocks everything but web tragic. Symantec used to sell a handy little box that could cripple a network with the flick of a switch.

I mentioned that Thunderbird is not my only client. All SMTP servers can be reached and accept e-mails from all other clients. The problem is isolated to this one Thunderbird client on one computer.

Turning off all firewall and anti-virus does not solve the problem. I am running absolutely no additional security software beyond whatever is in the OS, which is Windows 7. No updates of any software were performed within a week of the Thunderbird problem arising.

The router on our network (five PCs) is operating fine and no ports are being blocked. We can do everything imaginable on all PCs except send e-mail on this one Thunderbird.

I restored the backup of the Thunderbird profile from before the problem started and that has not worked. I guess I am going to have uninstall and reinstall Thunderbird. Or call an exorcist.

or try logging the connection to see what might be happening. Than we both might know more.

https://wiki.mozilla.org/MailNews:Logging#Windows

But make sure to change the example logging to the correct protocol. It is easy to forget.

Is it really only Thunderbird. Try telnet connection. https://docs.microsoft.com/en-us/exchange/mail-flow/test-smtp-with-telnet?view=exchserver-2019

IT is stated for Exchange, but SMTP should be universal really.