Just to make a comment: the error message may be true. The path from your PC to the Comcast server, receiving the message being sent and also the copy to be saved. That is two transactions, and the path when sent from web server is not the same, nor is the path to Yahoo the same. So, while there may be extenuating issues, the message is sometimes true.

David - thank you for your comment. I agree that the error message may be true; I guess my question is how best to troubleshoot this problem?

I am not an expert on email client/server communications, but my thinking that if the message is true then this is a very specific path/access issue, because a) the emails through Comcast/Xfinity do go through (they just don't end up in the Sent folder), b) if I get on the Comcast server and send an email, it gets copied to the Sent folder there, and then it shows up in the Sent folder on my PC, and c) if I get on the server and compose an email and save it, it goes to the Drafts folder on the server and later shows up in the Drafts folder on my PC. So it seems like there is communication back and forth, except in the specific cases of a) email that's sent from my PC not being copied to the Sent folder, and b) email that's composed and saved on my PC not being saved in the Drafts folder.

But, as I say, I'm not an expert, so maybe my thinking is off-base.

I am no network guru either; when then happens to me, I switch the Draft folder to save to PC. That eliminates the multiple network connections. That might be a temporary solution for you.

David - thanks for the suggestion. As it happens, everything is working fine now, so I'm all set for the moment, but I'll remember that I can do that if this happens again. Thanks for the tip.

I'm not sure if something that I did fixed it, or if it was just one of those things. A few weeks ago, I had trouble synching the photos from my iPhone to my PC (which has been working for years). A suggestion online was to disable iPv6 on my network card, which fixed that problem. Yesterday, I re-enabled it to see if that fixed Thunderbird, and it seemed to, but iPhone photo sync was broken again. So I disabled it, and photo sync was okay but Thunderbird was back to having problems. Then a few hours later, Thunderbird was working. So ... was it Comcast? Thunderbird? some weird PC setting? I'll probably never know, and I'm just keeping my fingers crossed for the future ... :-)