If you are migrating to Microsoft Exchange 2010, try to upgrade your client computers to Microsoft Office 2010 or Microsoft Outlook 2010 before Exchange 2010 server components are deployed. Outlook 2010 is the preferred and most full-featured client for Exchange 2010, and offers the only guaranteed seamless transition path. This way, users experience the full capabilities and features of Exchange 2010 as the new mailbox, hub, and CAS servers come online, and none of the co-existence issues I’ll describe in this article.

In the real world, you may need to support small or large groups of computers running earlier versions of Outlook for a short time or indefinitely. A takeaway is that, with some planning, thousands of legacy Outlook 2003 clients can co-exist perfectly in a large enterprise that has migrated to Exchange 2010.

Outlook 2003 and Exchange 2010

The oldest version of Outlook that runs with Exchange 2010 is Outlook 2003 (Service Pack 3). There were a lot of issues with Outlook 2003 support in the initial Exchange 2010 release. Full support for the Outlook 2003 client became available with Exchange 2010 in Update Rollup 3 (V3) to Exchange 2010 Service Pack 1 (SP1), released in April 2011. With a new installation of Exchange 2010 SP1, current through Update Rollup 6, you will have few issues other than those described below with your Outlook 2003 SP3 clients.

Outlook 2003 doesn’t support the Exchange 2010 autodiscover service.

Issue: Outlook 2003 will automatically detect when a user’s mailbox has moved to the Exchange 2010 side. If the user is logged in, they get a prompt to exit and restart Outlook, and are automatically connected to the new server. If the user is not logged in, the next time they do try and log in (as long as the old Exchange server is online) Outlook will get a pointer from the old server to the new server, and update the local Outlook profile automatically.

The support issue involves users whose computers were off or disconnected from a time before their mailbox moved, until after the old Exchange servers were retired. Also users that might have multiple Outlook profiles that they switch between, can end up with one or more offline profiles being marooned, i.e., they can’t use “autodiscover” to repair their profile, and their old server name is stale.

Fix: Simply update the name of the Exchange 2010 mailbox server in the Outlook profile.
Tip: If you deployed a load-balanced RPC CAS Array, the internal DNS name of the array is used as the Exchange server name in the Outlook profile.

Outlook 2003 users that open many shared calendars or mailboxes get a connection error.

Issue: Each mailbox or shared calendar that is opened in Outlook 2003 actually opens two or more network connections back to the Exchange servers. Exchange 2010 introduced a concept called Throttling Policy that prevents the email service from being bogged down due to excessive connections by any one user.

  • There is a limit of about 5-6 shared calendars or mailboxes that can be opened by a default Outlook 2003 user. Delegated users like admin assistants that manage the mailboxes and/or calendars of several managers are a typical user that needs attention here.
  • Outlook 2003 users that need to open many shared resources need to have a special Exchange 2010 Throttling Policy applied to them.

Fix:  Create a custom Throttling Policy that allows more concurrent connections, and run this Exchange 2010 PowerShell command for each Outlook 2003 user that opens a lot of shared mailbox resources:

Set-Mailbox -Identity “alias” -ThrottlingPolicy “ThrottlingPolicyName“

Tip:  Avoid the temptation to add all users to a custom policy. This can defeat the Throttling feature and induce server resource exhaustion.

Users with shared calendars saved in Outlook 2003 navigation pane get a connection error.

Issue: Outlook 2003 maintains hidden persistence messages containing information about the shared mailboxes and calendars, such as the user name and the server where the mailbox resides. These links are not always updated when server name changes.
Fix:  There are two ways to fix this on an Outlook 2003 client:

  • Remove those stale entries, sync the deletion with server, close and reopen Outlook, and re-add the entries again.
  • Close Outlook, and re-start with the /resetnavpane command line switch, for example: ‘outlook.exe /resetnavpane’. Then add all shared calendar entries back.

Tip:  The second method (‘resetnavpane’) is the quickest. Take a screenshot of the calendars before closing Outlook to jog your memory when adding them back.