Outlook problems "Updating cached messages"

[Labsy]

Hi,

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?

[Labsy]

I investigated this problem further and found out that above mentioned Outlook (desperate) slowliness approximatelly correlates with one user's mass mailing - he was sending 6000 mails out at one occasion, and around 7000 mails the second time.
Both times users with large mailbox sizes were exepriencing very degraded Outlook performance, for example, when switching views from one folder to another users were forced to wait up to 10 minutes!!! (Outlook was in "Not responding" state, or "Updating Cached Messages" was displayed on bottom-right.

On the other hand, I did NOT notice any CPU/network/disk I/O issues on server. Weird, huh?

Might it be Zimbra having problems with mass mailing?
My server is setup to allow 20 outbound simultaneous threads - is this too high?
Can I somehow DISTRIBUTE/PRIORITIZE mass mailings on Zimbra MTA level, for example:
- give priority to mails with less recipients, so mail with 1 recipient would have highest priority
- AND to limit outbound threads to 1 or 2 per second

Is it possible? Is it a good idea?