Opening multiple calendars with Outlook 2003 and Exchange 2010

Due to the way Outlook 2003 interacts with Exchange 2010, if a user on outlook 2003 tries to open multiple shared calendars, they may receive an error:

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.

This is because outlook 2003 has to open multiple mapi connections to exchange 2010 for each calendar, and in doing so, hits the default limit of concurrent connections set in the default throttling policy (20). This problem occurs due to Outlook 2003 dependencies on reference Mailbox Database support, which is not supported in Exchange Server 2010. Outlook 2003 clients must now reference the Exchange Server 2010 Address Book service when they open shared calendars.

Usually, restarting outlook provides a temporary fix, but as more mapi connections are created, the limit is reached again.

Using Exchange Shell, I created a new throttling policy with a maximum of 40 concurrent connections.

To create a new policy:

  • New-ThrottlingPolicy <policyname> -RCAMaxConcurrency 40

Check the details of the policy:

  • get-throttlingpolicy <policyname>

Check the user has the default policy applied already:

  • Get-Mailbox -Identity <username> | fl

To apply it to a user:

  • set-mailbox -identity <username> -throttlingpolicy <policyname>

Then test opening multiple calendars.

There is actually a registry key that sets a limit on the Exchange Information store, at 32 connections, so without changing this, clients will still be limited in the number of calendars they can open, but it looks like 32 connections is enough to open 5-6 calendars.

If you were to change these registry keys, they are:

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

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