I'm running ZCS OSE 6.0.6 on RHEL 5.4 x86_64. I know this system isn't up to date but our company takes a lot of care before upgrading anything.
Our clients are MS Outlook all configured with POP3 accounts.

We're experimenting a "lost messages" problem with things suchs as "I sent this e-mail but you didn't receive it".
To debug this situation, I disabled Message-ID duplicated deletion:

$ zmprov mcf zimbraMessageIdDedupeCacheSize 0

... and then I configured "keep a copy at the server" feature of the POP3 account in Outlook, so I can verify if a message efectively arrived to a user mailbox looking for at /opt/zimbra/store/...

Then this problem of "lost message" happened again: A message wasn't received by the user in his Outlook but the same message was found at /opt/zimbra/store/...

Is there a way to know why this message wasn't downloaded via POP3 by Outlook? Or maybe better, can I prove that Outlook is downloading this "lost message" but is deleting/missing it because of a Outlook's bug?

Maybe, is there any incompatibility/bug at Zimbra 6.0.6 that creates this problem with Outlook clients?

I hope some one can give some information or ideas to debug and find the responsible of this problem.