On Monday, 20 May 2019 19:26:20 UTC+1, Harry Bloomfield wrote:
Without my doing anything, the problem has gone away. Weird..

May have been something stuck on the SMTP/POP3/IMAP server which has got resolved.

Your email got sent, but the server didn't respond that it had been sent, so it still showed as unsent but wasn't there.

And now it's all synchronised itselves together.

Owain