I've used Thunderbird connected to comcast.net for months on a new laptop. No issues, and it worked fine - until I left my home WiFi and attempted to use WiFi from my home in France. At that point, I just got an error condition saying comcast is rejecting my login attempt. I've worked with Comcast technical support, with no improvement. They do not seem to detect any blockage of my IP address here.

Hmm, Comcast email is now provided by Yahoo. Have you completed to transition process to Yahoo? Have you tried using DNS over HTTPS for DNS resolution? It is in the settings of Thunderbird that might have some impact depending on what the cause of the issue is.

I thing the issue will actually be your antivirus program but I am just guessing and base it more on my distrust of them as a technical solution. Do I wish to give access to all on my unencrypted communications to XXX government. (nation were the antivirus vendor is "resident".) All of the third party products obtain such access and like we keep getting told. Companies corporate citizens have to comply with information request from their legal system.

Personally only use Microsoft Defender these days. Everything else is so busy trying to be more they mostly fail at being anything useful at all.

Thanks for the suggestion. I have DNS over HTTP set to Max Protection, but it does not log in to comcast. I've tried turning DNS over HTTP off, and that does not work, either.

I'm just using MS Windows Defender as antivirus protection. I've tried running Thunderbird in Troubleshooting Mode, but it still fails. I don't know how to see exactly what Thunderbird is sending as login credentials, although they appear to be set properly.

I'm not sure about the change to Yahoo, but given that it worked fine until I left home, I'm not sure if that's the issue. I'm looking into whether there is anything I need to do for that change.