So you still have issues opening many shared Exchange 2010 calendars using Outlook 2003?

Even though you tried all the steps in the “Opening multiple shared calendars & additional mailboxes” section of the “Concern: Is having Outlook 2003 clients going to prevent me from deploying Exchange 2010?” TechNet Wiki article, I wrote some months ago.

So during a recent Exchange 2003 > Exchange 2010 transition at a customer that only had Outlook 2003 clients and relied heavily on shared calendars, I discovered an additional issue. Even though I had configured the value of the “RCAMaxConcurrency” setting in the default throttling policy to unlimited ($null), some users still saw the dreaded “The action could not be completed. The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete this action. error message. In addition, event 9646 with a description similar to the following was logged in the application log on the Mailbox servers in the organization:

“Mapi session “00cc8dde-64d7-4353-8050-00fc2057aae3: /O=xxxx/OU=xxxx/cn=Recipients/cn=John” exceeded the maximum of 32 objects of type “session”.”

So after some researching and communication with two Exchange Escalation Engineer buddies (Steve Swift and Will Duff) from the Exchange Enterprise Support team in Charlotte, I finally nailed the issue. The issue wasn’t really related to the Exchange RPC CA service, but a limit set on the Exchange Information Store. Yes same limit as you probably have seen in previous Exchange versions.

To fix it, you need to increase the limit for MAPI sessions using an Exchange Information Store registry key. The keys to use are covered here:, but when I tried to add the “szMaxAllowedSessionsPerUser and/or “szMaxAllowedServiceSessionsPerUser”, I still saw 9646 in the app log.

Guess why? yes the registry keys are actually listed with wrong names in that article. Instead of:

  • szMaxAllowedSessionsPerUser  
  • szMaxAllowedServiceSessionsPerUser

You need to use:

  • Maximum Allowed Sessions Per User
  • Maximum Allowed Service Sessions Per User

And then everything worked as expected…

Hopefully the TechNet page is updated soon.

Until later,

Henrik Walther
Technology Architect/Writer
MCM: Exchange 2007 | MVP: Exchange Architecture


About The Author

Leave a Comment

Your email address will not be published. Required fields are marked *

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.

Scroll to Top