Date/time problem with some Zimbra users

[tenpagyatso]

Three of my 1000+ users are reporting a problem with inbox messages displaying improperly re: their arrival time. I haven't found anything in their Prefs to show why this would happen.

During the day they'll find out from someone an email message that was sent to them hours ago hadn't arrived yet. When the users look at their Inbox and don't see the offending message they manually Get Mail and the message in question shows up but from a significantly earlier time in the day. But the date/time stamp of these messages does sync with when the sender reports having sent the messages.

On first glance I was chalking this up to user error but my director has sent some test messages from his Gmail account and has experienced this. None of my tests with my Zimbra account have shown this problem. There is no correlation between the accounts experiencing the problem other than all the accounts are in the same COS.

Can anyone give me some direction as to where to start to look for what might be causing this? I've already verified the OS clock and it's tracking properly. All the Zimbra users in question are using the web client.

Thanks in advance...