From time to time over the past few months Apple Mail has been refusing to send some of my email, generating rather cryptic error messages. I called Apple support earlier today and got help so I could send from my personal domain, but the problem reappeared a couple of hours later. This time I found the problem – I use several email accounts, and when I looked at Mail>Settings>Accounts>ServerSettings I noted that the Outgoing Mail Account was blank. I put in the proper account names and that seems to have solved the problem.
Now that I look back, I realize the problem popped up when I was trying to reply to incoming mail from some of the lesser used mail accounts. When Mail found the Outgoing Mail Account blank, it tried to send through the default outgoing mail, which refused to send it because it was being sent from a different email.
I don’t know how the Outgoing Mail Account box was erased. The two most problematic accounts were gmail accounts, but it may have happened in the process of moving my personal domain email to FastMail. In any case, I thought I would share a solution for this maddening problem