We're using qmail as our MTA, and Courier to handle IMAP and POP3 service, with the "authpam" module providing authentication for Courier, as we use a centralized LDAP/Kerberos system to handle authentication (and I didn't see an easy way to get Courier to talk to Kerberos).

The problem is thus: Users will randomly have authentication problems; their mail clients will prompt them to retry, and things will work the second time. The logs on the KDC (auth server) don't even show Courier trying to authenticate the user on these failed attempts; this happens both with and without the auth daemon.

CPU load/etc on the mail server is nominal.

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