Are all your mailbox archiving task set to run at the same time, synch at the same time? I remember you were asking about running synch for all of your task at the same time, is that right? I did say that it is not a good idea.

https://www-secure.symantec.com/connect/forums/any-way-synch-all-mailboxes-under-multiple-exchange-mailbox-tasks-once

You need to schedule your task out evenly because you are running 1 EV server to host 8 active tasks. How many active users/mailbox have you got in your exchange environment?

Also go through the MAPI troubleshooting one example technote is here:

http://www.symantec.com/business/support/index?page=content&id=TECH49069

I would also look at clearing out your Outlook profile more frequently form the technote above. 

Many Thanks LCT, yes after your suggestion we have changed sync to differrent hours throughout the day for differrent archiving tasks,

Number of EV enabled total users about 2000 at the moment spread across 4 active servers in Exchange 2010 DAG, 4 servers are passive.

Will change the MAPI profile expiry time to 1 and see what happens.

Do I need to look at setting registry key "NSPI max sessions per user" in Domain controller? 

Thanks again

if you are running Windows 2008 R2 domain controllers yes then I would advise you to.

Can I also manually remove All profiles, would that have any impact?

yes you can delete them manually...when the archiving task runs they will get created...hence you need to remove regularly using the above.

Also I would advise you to schedule for EV services restart on a daily basis this will help clearing out cached connection etc.

We went through all troubleshooting steps, MAPI stguffs, we have set profile expiry to 1

Also Manually crealed all profiles 

We have NSPI max sessions per user to max 0xffffffff ....

Exchange trottling policy is set

Mailbox Sync and Schedule for each tasks now been scheduled during differrent time slots rather than running all at the same time.

Still some archiving tasks failing randomly, sometimes restarting task controller service fixes the issue, but sometimes some tasks still keeps failing. We found that from EV server if we create a profile for that specific system mailbox for failing tasks and open outlook using that profile, then the task starts fine again.

Any clue please? What can we check to further troubleshoot this issue please?

Many Thanks

Ok, I have set the following two reg keys, all archiving task seems to be started fine, let see what happens in next few days

HKEY_CURRENT_USER\Software\Microsoft\Exchange\Exchange Provider

Value name: Closest GC

Data type: REG_DWORD

Radix: Hexadecimal

Value data: 0x00000001

HKEY_CURRENT_USER\Software\Microsoft\Exchange\Exchange Provider

Value name: DS Server

Data type: REG_SZ (string)

Value data: FQDN of the global catalog server

If that still gives you problems then I would log a case with Tech Support to do indepth tracing of the problem.

Use DS Server and specify your CAS as the GC
Once done, if it occurs check the event viewer on the CAS server for clues

If we have a CAS Array do we specify the CAS array FQDN or just any specific CAS Member?