This happens when there is too much traffic for server to handle the mail. Thunderbird on your PC sends a copy to recipient, which goes through your email server, and then sends the same message to the same server to be stored in your SENT folder. This second send is what is sometimes prevented because the server is overloaded. This only happens for IMAP accounts. If this happens frequently, you may want to report this to your email service provider.

Hi David

Thanks for your answer. My settings are such that I save sent messages to a local folder, not the cloud. Would that change your explanation?

Yes. Clearly different. If TB is working normally and this was a one-time event, I would write it off as a rare conflict within the process of saving email. If it happens frequently, it may be that the sent folder becomes corrupt and a repair may suffice. I regret I have no specific advice on this.

Thanks for your answer. Perhaps if I compacted the folder? Maybe that would help?

Good idea. You might also see if your antivirus is examining the sent mail. If so, consider turning it off. Although not related to your issue (since I use IMAP), I find I must repair the sent folder almost daily, due to its becoming corrupted. It's something I just live with. Good luck in your compacting the folder. There is an addon, Xpunge, that might be useful, simplifying the process of periodically compacting folders.