I'm the administrator of a ZCS OSE 6.0.x integrated with MailScanner running on RHEL 5.4 x86_64. The system is working fine but several users are reporting that some sent messages never arrive to the inbox of the destination user.

According to that I asked for info of those messages like:

- Sender address
- Destination address
- Date and time

With this informationn I proceed to look at /var/log/maillog and I find that messages that was successfully delivered according to postfix logs like:

relay=mail.domain.com[]:7025, delay=3.9, delays=3.5/0/0/0.43, dsn=2.1.5, status=sent (250 2.1.5 Delivery OK)

This messages just tells me that Postfix (Zimbra's MTA) delivered correctly the message to the Zimbra's POP3/IMAP server (vía LMTP?), right?
So I asume that this message wasn't deleted/retained/quarantined/etc... by MailScanner nor Postfix.

Then I try to look for the messages at the mailbox folder of the user (at /opt/zimbra/store/...) but I don't find the message because the user already downloaded and deleted it vía their MUA (MS Outlook) using POP3.

That's all I can do to "probe" that messages aren't lost. But users still keep claming that some messages get lost (occurs randomly).

I suggested to the local administrator of this company that verify things such as:

- Junk folder of the MUA (MS Outlook)
- Custom rules/filters of the MUA (MS Outlook)
- Antivirus quarantine & report
- Even suggested to stop using the MUA and just use Zimbra Web (no success, users can't stop using their favorite crap called Outlook)

So now I'm not sure who's responsible of this lost messages. Is it Zimbra? Is it the MUA? Is it the network causing errors on communication between client and server?

I would like to know if there's a way to turn on a debugging mode of Zimbra of the POP3 server, so I can know what messages got downloaded by whom.
It would be great if I can have a log of every message stored at POP3 server of Zimbra and when was downloaded via POP3.

I hope someone can help me with some ideas.