Previously working email alias fails with error 450? [SOLVED]
Due to a change in domain host, the security key for my custom domain could temporary not be resolved by DNS. This situation only lasted for ~6 hours.
The result was that my email alias "@my-domain.com" gave an error "450 4.1.8 Sender address rejected: Domain not found". Luckily this is only a temporary error and smtp servers will keep on trying to deliver the message. So you're not necessarily losing emails (in the short term).
First, I made sure the DNS was OK for my domain with the proper security key, mx servers and spf text. I made sure the DNS was propagated worldwide via https://www.whatsmydns.net with the proper info (key, mx, spf).
The email alias kept on failing, even after 10 hours of succesful worldwide propagation.
Deleting the email alias from the webportal didn't help either. The custom domain can be deleted and added back. But this doesn't change the situation.
In the end, the situation resolved itself after 24 hours. Not sure why. I'm guessing it has to do with some backoffice internal mailbox.org process.
So anyone in the same situation: make sure the dns is OK, verify it is properly propagated wordwide and then wait for 24 hours ...