I have a zimbra server with a strange situation. Some users are reporting me download error with ms outlook. Apparently the pop service cant be contacted some times; but the sending process work perfectly.

I discard dns problems (pop.server.com resolv without problem). I am monitoring the pop service availability and report 100% uptime. When a user report download error, i try to connect over telnet the port 110 in pop.server.com and work perfectly, but outlook continue showing the error. Some minutes later, all work fine again.

How can i follow the error? The pop service is running permanently, the error is not at the same time for all the users, connect again without any change, and the pop service is alive permanently.

They was use Zimbra Comunity 6.0.2 and was upgrade at last weekend to 6.0.6, but the problem continue.

Any sugestion to detect the problem?

Thanks in advance.