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Today, I suddenly started getting error messages when sending mail to my user accounts. Some messages go though and some get rejected. The rejection message is 550 5.1.1 no such user here". But it is totally random as to which user it happens to and when it happens.
In fact, I can't even send a message to myself when I am already logged in as myself. I get the same error message in the log, but I do not get the undeliverable message in my email as I would if I was sending it from outside my domain.