Is your smtp server mobile.charter.net? I encourage carefully reviewing your smtp server setup and compare to the older PC. Your error message is usually a misconfigured smtp server.

Thanks, David, for your suggestion. I have compared old to new, and they are identical.

This is not an area of expertise for me, but there have been several similar posts regarding Charter and inability to send messages. Would you mind posting a screenshot of SMTP server setup? That may open a window.

Here is the stmp server info. I have read a bunch of posts about this problem. None have furnished any new information. I am beginning to wonder if there is something in windows 11.

I wish I could give concrete suggestions, but I struggle with Charter accounts, since the proper server seems to be inconsistent. . Yesterday, a user posted that he was advised by Spectrum to use smtp.charter.net, but he chose mail.charter.net because the one recommended didn't work. You might try changing options, such as port 465.

Try mail.charter.net for the smtp:

https://support.mozilla.org/en-US/questions/1398181#answer-1553359

So far, no combination of mail, smtp, or mobile as the prefix to charter.net has worked. I have also used 465 and 587 as the port with all the different prefixes. Is anyone else using at&t fiber for their provider and spectrum email?

Hello bobcaine

you said in your first post that mail worked fine on your old computer. But is it working fine with another Thunderbird installation, on the same network (that is in the same place with the same Internet box), or is it working with the provider webmail ? If it's webmail, it's not surprising, but it would defy logic that a server problem occurs with one computer and not on another. This is a provider problem. These guys don't want their users accessing their mail server using different software. It would be the same with any other mail software, Oulook or whatever. I tried to do a test connect from 2 different IP addresses and the reply was the same; the IP address is not trusted, even if the whole purpose of connecting to a mail server with a login and a password is to allow sending mail from networks not controlled by the mail server operators. BTW, the only port allowed is 465 with direct TLS (Starttls do not work). Asking for a reverse is even more insane. No customer IP address will have a correct reverse. It's hopeless. This ISP is hopeless.

Hello again bobcaine :-)

I had a flashbulb moment trying to understand why an ISP could behave like that. Did you in the recent past received a demand from them asking to change your password ? Because it is reminding me of a recent adventure in my own country of a big mail provider that blocked also all access from external tools, it was (not) explained but it's pretty clear IMO that they were hacked. They first blocked all access from alien countries, and then all accesses not done by external tools (only webmail) and finally resetted the passwords for all of users. Only after weeks of this rigmarole was normal access by external tools allowed again.