I boutght a PC through Newegg that was supposed to be great. But somehow I find a way to gum them up. This has not had the problem right along. It's only been in the last couple of weeks. There are some other, less annoying, glitches with Thunderbird. That's why I'm considering switching out of it to EmClient. Advise?

Looking at your OP again, the problem may not be related to your anti-virus software at all.

I regularly see the same effect with Yahoo and AOL accounts, where copying the message to the Sent folder fails initially. After a timeout, when prompted to try again, the second attempt usually works.

I don't think the problem is with Thunderbird, I believe it's something with the server.

I'm afraid there is no solution for your problem at this time. Gmail usually works fine here, so you may think about an email provider change.

So you're certainly saying that switching out of Thunderbird will not likely solve it. Can my server fix it, you think? Thanks yet again for you time and effort on this. It's not something crucial but annoying for sure. Time consuming. It does exactly as you describe in this last post.

I don't know if your email provider can or is even willing to try to fix it. But that seems to be the most likely scenario.
Another factor may be your Internet connection. If it's slow, with high latency and jitter, this may contribute to the problem.
As said before, there isn't an easy fix. You may try to experiment though.

I have DSL, maybe not the fastest but not usually slow at all. I called the server and we tested a few times. Could not duplicate problem. I'm going to try a different email program. I hope I can export everything from Thunderbird to EmClient. Thanks for everything.