Phil Lavin writes:

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This is a very weird problem. I did a standard apt-get upgrade on Debian and got Courier IMAP 4.9.3-4. On upgrade all worked fine, however within 2 hours the imapd stopped acknowledging successful logins. When it works, it returns “1 OK LOGIN Ok.”. When it stops working, it returns nothing. The state, as I know it, is thus:

Check for the number of concurrent connections at that time. The server enforces an upper limit on the number of concurrent connections overall, and the maximum number from a given IP address. When the limit has been reached, the server stops accepting any more connections.

·         It will acknowledge and log failed logins

·         It will not acknowledge or log successful logins and it will not accept commands after a valid login is supplied

If, for example, you're running NFS and the kernel hangs the user process on a stalled NFS request, that's still an active connection, for all intents and purposes. Until the process terminates, the connection exists. After some time elapses, a sufficient number of dead zombies made the number of active connections reach its maximum. That would be another possibility.

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