What happens in Windows safe mode (similar to TB safe/troubleshoot diagnostic mode)? Who is the mail provider? Some of them copy sent messages automatically, which must be accounted for in TB.

Thanks for the suggestion, but I'm reluctant to mess with starting in Windows safe mode. There are no other problems with my Windows setup and applications. All other mail and internet functions are normal. My mail provider (The Phone Coop) is reliable and I have spoken to their tech support. They have not had reports of problems with their mail servers from any other customer.

I don't see any possible negative outcomes in running Windows in safe mode, temporarily, to test the effect of startup apps, especially antivirus, on TB performance, as AV is the most likely cause of poor performance. Or, simply remove (not disable), temporarily, the current AV, if it's not Microsoft Security, and see if there's an improvement.

Thanks, point taken. I restarted Windows in Safe Mode, but the long send delays remained the same. My AV is Bitdefender. So presumably that's not the culprit.

Does this happen with all accounts, or just one? If you have other accounts such as gmail, can you test sending with them? You could also add the account to the W10 Mail app and see if the issue remains.

Thanks! Today the problem has mysteriously disappeared. I suspect some change at our email provider, although they previously said it could not be them. Who knows? Fingers crossed, and thanks for your helpful suggestions.

For me, gmail said it is not their problem. After few days of problem this morning it was fine for 35 posts. This evening the problem returned.