Thanks for the quick answer.
I realised, that the error didn't occur since quite a while, opposed to what our client suggested. Back then I activated the two workarounds (imap_client_workarounds = outlook-idle delay-newmail) and increased mail_max_userip_connections for IMAP.
Is it possible that those could have improved the situation?
For now I don't have a way of reproducing the problem, so I will have to wait for an error to happen. Until then I will consider the problem as "Solved until happens again ;-) ", since the last error occured a while back (as mentioned above).
Mar 6 08:26:31 mail02 dovecot: IMAP(user@example.com): fdatasync(/data/vmail/example.com/user/dovecot-uidlist) failed: Input/output error Mar 6 08:42:29 mail02 dovecot: IMAP(user@example.com): Maildir /data/vmail/example.com/user: Expunged message reappeared, giving a new UID (old uid=1522, file=1326961561.V15I4d8562M567017.mail02:2,Sad) Mar 6 08:42:29 mail02 dovecot: IMAP(user@example.com): Maildir /data/vmail/example.com/user: Expunged message reappeared, giving a new UID (old uid=1523, file=1326705103.V15I90105M613353.mail01:2,Sad) Mar 6 08:42:29 mail02 dovecot: IMAP(user@example.com): /data/vmail/example.com/user/dovecot-uidlist: Duplicate file entry at line 4: 1326961561.V15I4d8562M567017.mail02:2,Sad (uid 1522 -> 1598) ..
My suspicion/speculation what happens is the following: Multiple users are accessing the Mailbox from their offices (all on the same server), one (or more) uses the Webmail or accesses the Mailbox from a different IP. Somehow this leads to problems with Locks on NFS, which leads to the crash.
Yes, most likely this is what's happening. Although your errors are more severe than what normally happens. I guess your NFS server is also partially to blame (microsecond resolution timestamps are at least helpful).
I had a quick look a tour NFS (NetApp), but didn't find anything useful. In case the problem persists, I will check with the coworker responsible for NetApp.
I will check what sort of locking is used by postfix, since I'm not sure if postfix and dovecot are configured to use the same mechanisms. If not, I think it could be part of the Problem.
I have no idea how to solve this problem and any help is greatly appreciated.
The only way to fully fix this is: http://wiki2.dovecot.org/Director
Unfortunately that is not an option right now, but I will keep it in mind.
Thanks again.