Sudden "Sender address rejected: not owned by user"

I set up a new server last week for 3 domains I own, and it was working flawlessly until today.

Now incoming email just disappears as if it never existed, and outgoing email gives this message:

An error occurred during sending the message. (553 5.7.1 : Sender address rejected: not owned by user" no matter what address I’m trying to send from (even my admin account).

Luckily I hadn’t put this server into production yet.

Any ideas what would be causing this?

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