we have ZCS with approx. 20 domains and total of 200 users. Maximum mailbox size is 5 GB, but mostly users have up to 800-1000 MB of mail, not more.
From time to time, let's say every 1-2 weeks, users report very very slow response of their Outlook clients. Changing between folders is very slow, they all are connected with IMAP protocol.
Most users see "Updating cached messages" notification in lower right corner of outlook at that time.
When I look at either ZIMBRA server, or network, nothing is busy, nothing is above normal - CPUs on server are on approx. 14%, network graph is like a carpet, just few pixels above floor...nothing abnormal at first sight.
the common for ALL users having problem is OUTLOOK, any version, from 2003 to 2010. And common is also IMAP access.
Any idea where to start debugging?