Apple mail error "message could not be sent because your account does not have a preferred outgoing mail server"

I can access mail absolutely fine from pretty much everywhere. The only issue I have is that Mail on my iMac seems to forget the server details if left idle for a long time. Just quitting mail and restarting it always fixes it, so I guess the workaround is not to leave Mail open - but I expect Apple software to "work".

The fact that I don't have to change anything to "fix" the problem (apart from restarting Mail) points to a "system" type problem to me.

Drafts also son't save if you choose that option - so again I think it is a "system" issue.

I can only remove the account from "Internet Accounts" as it is an iCloud account. Have tried that - Mail launched with empty folders. Re-enabled it. So will wait and see.

I left the machine on overnight.. this morning I sent a test email to myself and it worked OK - no error and no need to quit/restart.. Fingers crossed that was all it was!

Thanks :)

Actually, I spoke too soon. I sent a "proper" email the next day and got the same issue so removing the account didn't actually work!

Also - if I choose to save the email as a draft... to doesn't get saved.

Nothing else listed other than iCloud...

I did have Clam XAV running - have uninstalled that and the same thing happens after a couple of days, so it's not that either. Nothing else of that ilk running.