That sounds like a classic case of an antivirus product that purports to scan mail dropping off to sleep and only waking up after the timeout occurs. This is based on you simply repeating the failed step working. It is the only thing I can think of that would change from one click of send to the next.

Thanks for your reply. This just started happening with the upgrade to the SuperNova version 115.xx. Prior to that it was not an issue. I'm using BitDefender and will change the setting to determine if that is the issue.

I disabled outbound scanning of email in my antivirus software, BitDefender, and it didn't resolve the problem. It also doesn't matter if I am directly connected to the internet or on a wi-fi connection to my home network,

As I stated before, this problem didn't exist until the latest upgrade to Thunderbird.

I specifically mentioned mail scanning, but my understanding of bitdefender is it has all sorts of nasty bits that are supposed to be helpful and can only be deemed a problem when they do not work like clockwork. Including scanning all encrypted connections, an anti spam toolbar that has caused much difficult in the past as well as a habit of forcing updates before the development team even authorize them for existing users. So forgive me if I am not convinced that yet again an anti virus product struggles following a software update of Thunderbird.

I suggest you try your operating system in safe mode with networking and see if you issues continue while in that mode.

Safe mode instructions for each operating system: Windows 10/11, Windows 8, Windows 7, OSX