Why is my mac.com outbound email no longer working?

Hi, I have had four mac.com email aliases in use for about 15 years. Today, my 2017 iMac running Mail on Mojave suddenly stopped allowing mail to be sent from one of my aliases. I received the following message:

The sender address****@mac.com was rejected by the server p72-smtp.mail.me.com

I haven't changed anything, I have deleted iCloud accounts and added back - no change. Apple servers appear to be up.

Only one of the aliases is causing this problem. The others work fine. I've tried restarts etc. nothing changes.

If i click the "edit SMTP Server" button it brings up a window in settings, but there are no SMTP server settings to check. ie. everything is blank. If I click "edit message" and change the outbound address to one of my other aliases, it works.

Any thoughts?