Looking at mail server log, I caught a very obscure message :
Command stream end of file while reading line user=user1 host=[10.10.11.12]
And user1 is not receiving the mails sent to him. They are in a mailbox, but client can not get them to his host machine, and his user+pass combination is checked and is correct.
After some time spent searching, here is the answer :
“This message occurs when the session is disconnected without a proper LOGOUT (IMAP) or QUIT (POP) command being received by the server first.
In many cases, this is perfectly normal; many client implementations are impolite and do this. Some programmers think this sort of rudeness is “more efficient”.
The condition could, however, indicate a client or network connectivity problem. The server has no way of knowing whether there’s a problem or just a rude client, so it issues this message instead of a Logout.
Certain inferior losing clients disconnect abruptly after a failed login, and instead of saying that the login failed, just say that they can’t access the mailbox. If the user isn’t reporting a problem, you can probably ignore this message.”
As far as my user1 is concerned, it meant reinstallment of his Outlook….
A very good location to see such IMAP log lines is here.