RCA: Occasionally, Users Cannot Open Mailbox using Outlook

Exchange Server Enterprise 2010 SP3, RU8 v2

Every month or two, users whose mailbox resides in an active database on server are unable to open their mailbox using Outlook.

Is anybody familiar with this situation? Is it possibly related to events 4999 or 9646 on Mailbox? 1006 on CAS? The new "2010 RU 10 for SP3" may address 4999, but could it address this issue for users?
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Mark BillExchange, AD, SQL, VMware, HPE, 3PAR, FUD, Anti MS Tekhnet, Pro EE, #1Commented:
what size are the mailboxes?

roll up 10 should definetly be installed.

please ensure the client versions of outlook have the latest service pack and updates installed.

try checking for corruption on the mailboxes. http://www.falconitservices.com/support/KB/Lists/Posts/Post.aspx?ID=143

also try checking the database for errors too http://msexchangeguru.com/2009/07/12/exchange-database-recovery-using-eseutil-commands/

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kontrariankidAuthor Commented:
Thank you Mark... or is it Bill??

I will have to get the mailbox sizes.. that might make a lot of sense to ensure clients are updated. I have been focusing on the server side, working on ExPerfMon.ps1 to get RPC/CAS data.. seems that RPC Requests are overloading server.
Mark BillExchange, AD, SQL, VMware, HPE, 3PAR, FUD, Anti MS Tekhnet, Pro EE, #1Commented:

first run a mailbox scan.

then run an eseutil against the db
Mark BillExchange, AD, SQL, VMware, HPE, 3PAR, FUD, Anti MS Tekhnet, Pro EE, #1Commented:
you said it was just two mailboxes right? now it's the server?

I'd it a vm or physical?

can I see the performance graphs please?
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