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Resolving a MAPI error message in Exchange Server 2003

Find out how to resolve an error pertaining to control the number of connections per user during MAPI sessions in Exchange Server 2003.

The following error message below keeps appearing on our Exchange 2003 server:

MAPI session /O=QCA/OU=London/cn=Recipients/cn=KhanS exceeded the maximum of 32 objects of type "session"

We had the same issue last year and a LAN accelerator was the problem. However, I don't think that this is still the case. How can I solve this problem?

The first thing you should do is speak to your KhanS user to figure out what they're doing on the client side that is hitting your Exchange server with all these MAPI sessions. If they are a developer, find out if they are doing any coding around Exchange. If they are an end user, check to see if they have added a second mailbox to their MAPI profile. If so, find out how large this mailbox is, and whether it contains a large number of folders.

I imagine this behavior started when you installed Exchange Server 2003 SP1, which throttles MAPI connections to 32 connections per user. If the user is accessing a large mailbox, disable cached mode to see if this behavior ceases.

If neither of these suggestions point to obvious issues, check to see which MAPI clients are running on KhanS's computer. If you're certain that KhanS requires more than 32 MAPI connections, then you can increase the Maximum Allowed Sessions Per User registry key on your Exchange server. The key is located in the following path:

HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesMSExchangeISParametersSystem

If the Maximum Allowed Sessions Per User key doesn't appear, create it as a new DWORD value.

Next, set Maximum Allowed Sessions Per User to a decimal value greater than 32.

I'd suggest trying this in a lab first, backing up your registry and taking other precautions as you normally would when changing your production Exchange server. Once the change is made, recycle the MSExchange Information Store Service and monitor regularly to ensure that the issues have gone away.

Do you have comments on this Ask the Expert Q&A? Let us know.

This was last published in April 2010

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