Have you checked with Comcast, such as about the certificate? The fact that it works sometimes indicates a problem somewhere in the internet. You aside comment on Gmail was puzzling: POP accounts work fine with Gmail.

I have not had internet service with Comcast in several years since switching to Verizon Fios. Verizon allowed me to keep my Comcast address because it had just ditched its Verizon.net service. Should I approach Verizon with a request for an updated certificate instead?

Open the error console (Ctrl-Shift-J). Is there anything related?

What version of Thunderbird are you using?

Should I approach Verizon with a request for an updated certificate instead?

What are you talking about?

I'm using Thunderbird 115.9.0. The Ctrl-Shift-J command doesn't bring up the error console in this version of Thunderbird. I just tried changing my TLS version from 3 (apparently set by the latest Thunderbird update) to 1 in the Config. Editor. No change; Thunderbird still is blocking my emails or delaying their download by minutes if not hours.

My reference to Verizon was due to the fact that I can no longer count on Comcast to provide me with a certificate that Thunderbird will accept (I refer you back to the screenshot of the TLS error message, which referenced the certificate as a possible cause of the error).

I greatly any other suggestions you have. Thanks.

The Ctrl-Shift-J command doesn't bring up the error console in this version of Thunderbird.

So what happens when you press Ctrl-Shift-J? It certainly brings up the Error Console here with TB 115.9.0.

I just tried changing my TLS version from 3 (apparently set by the latest Thunderbird update) to 1 in the Config. Editor.

I'd not mess with TLS settings unless you know exactly what you're doing.

My reference to Verizon was due to the fact that I can no longer count on Comcast to provide me with a certificate that Thunderbird will accept

The screenshot with the error message you posted is not very specific. So it's too early to blame Verizon for using a bad certificate. Is that the only error you get? I'd expect more information in the Error Console

If you feel like to request Verizon to update their certificate - good luck with that.

I'm still not sure though why you're first talking about Comcast, and then mix in Verizon.

Also I'm wondering why there are more than 10 Comcast SMTP servers set up in your Thunderbird profile (3rd screenshot).