Cannot open mailbox from more than 5 PC's

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I have a strange problem with a user that is logged in simultaneously on 7 to 8 different PCs.

All clients are Outlook 2007 SP2 / Windows XP SP3 and he is connecting to his regular user mailbox on the Exchange 2010 SP1 server (single server with CAS/HT/MBX/UM roles installed). The user is running non cached mode and without Outlook Anywhere / RPC over HTTPs configuration or in other words a plain old MAPI profile. Cached mode is not an option due to a 3rd party application (Paradox) that uses MAPI send.

When he opens Outlook on the first 4 or 5 PCs he has no problems but when he tries to open Outlook on the next 2 to 3 PCs he gets an error message saying that the Microsoft Exchange Server is unavailable. When Outlook is closed on some of the PCs that already has Outlook running then he is able to open Outlook on the other PCs. In other words they all work individually but not all at the same time.

It obviously seems like some kind of session limit so I have tried the following without any luck:

On the Exchange server in HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSexchangeIS\ParametersSystem I have added the following registry keys:

Maximum Allowed Service Sessions Per User = 64 (decimal)

(found on

and from :

szMaxAllowedServiceSessionsPerUser = 64 (decimal)

szMaxAllowedSessionsPerUser = 64 (decimal)

When that didn't help I also tried adding:

szMaxAllowedThreadsPerSCT = 5000 (decimal)

szMaxRPCThreadsPerMDB = 5000 (decimal)

(the server only holds 37 mailboxes)

and after that even:

szDisableSessionLimit = 0


Disable Session Limit = 1

(last one found on

The Exchange server logs does not indicate any errors/warnings when the user tries to connect on a failing Outlook client. Get-LogonStatistics for the user shows approximately 25 to 31 connections (never saw it above 32).

Nothing of what I have done so far has solved the problem so I was wondering if anyone should have experienced a similar problem or could give som advice on how to troubleshoot it?

(I think I might need to enable some additional logging on the Exchange Server - but I am unsure which / where / how)


Great suggestion!

The parameter " RCAMaxConcurrency" looks promising:

" The RCAMaxConcurrency parameter indicates how many concurrent connections an RPC Client Access user can have against a server running Exchange 2010 at one time."

It seems that the default throttling policy sets it to 20. Will try to raise that and see what happens.


I have just run the following commands:

Get-ThrottlingPolicy | Set-ThrottlingPolicy -RCAMaxConcurrency 40
Get-ThrottlingPolicy | Set-ThrottlingPolicy -OWAMaxConcurrency 10
Get-ThrottlingPolicy | Set-ThrottlingPolicy -CPAMaxConcurrency 40

(Doubled from the default values of 20 + 5 + 20)

Actually none of the new settings seemed to make a difference right away but after 10 to 15 minutes it seems like it has solved the problem. Not sure which one did the difference though


End user reports that Outlook now works on all PCs simultaneously.

Andreas: Thanks for guiding me that way!
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