Confusing SMTP problem

I have a customer with multiple domains, one of which has over 120 email accounts on it. None of his other sites and none of the other sites on the server are experiencing this, so I'm quite confused.

Periodically, he starts getting authentication failed error messages when trying to send emails. Creating a new account under the domain appears to reset something or, well, it fixes the problem temporarily. I've been through the issue over and over again, off and on for a few months now and had thought it was corrected after 3 weeks with no communication from the customer. Just heard from him and the problem recurred 2 days ago.


Anyone have any clue about this one?


EDIT: Exact error message

The email you sent to:
"******@***.***.**"
was not transmitted.
Reason: 535 Incorrect authentication data

 

 

 

 

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