Edit 23-5-2013: Added Headers as getting too long
Edit 23-5-2013: Added reference for 940012
Edit 24-7-2013: Added reference for 2557323
While Outlook 2007, 2010 and now 2013 offer many, many, many * improvements over the older Outlook 2003 client there are still many, many, many * large enterprises that use Outlook 2003. At this point in the lifecycle of Outlook 2003, customers should be looking to migrate to a newer version. Most customers that I talk to are doing that; typically in conjunction with a desktop refresh. Exchange 2013 will not support the Outlook 2003 client, and in addition there are upcoming support expiration dates that we should all be familiar with:
Outlook 2003 will transition out of extended support on 8th of April 2014
Exchange Server 2003 will transition out of extended support on 8th of April 2014
Windows Server 2003 will transition out of extended support on 14th of July 2015
Windows XP will transition out of extended support on 8th of April 2014
The Lifecycle site’s FAQ has more information and details on support options if you are not able to complete your migration prior to the end of support dates. And while you are there also take a look at the date that Exchange 2010 SP2 will transition out of support: Exchange 2010 SP2 will transition out of support on 8th April 2014. Why you may ask? Well as per the lifecycle policy since Exchange 2010 SP3 shipped then there is a 12 month period for customers to move to the new service pack.
For those organisations that are still using Outlook 2003 there are some considerations when coexisting with Exchange 2010. They are listed in no order of priority, and I’ll come back and periodically update this listed based off comments to the blog and also add other issues that I see and hear about. Please do not read this as an definitive list, consider it more a public bookmark that we can share 
"Cannot open your default e-mail folder" error when users try to open their mailboxes in Outlook after migration to Exchange 2010 – users unable to logon to Outlook after migrating their mailbox to Exchange 2010. Exchange 2010 OWA works OK. This is due to duplicated addresses.
Exchange 2010 Throttling Policies
Outlook 2003 connects to Exchange differently that later versions. So when running into issues try to isolate by comparing O2003 results with O2007, O2010 and OWA. For Example you may see Outlook 2003 running into issues with Exchange 2010 throttling policies.
This problem occurs because of Outlook 2003 dependencies on reference Mailbox Database support. This 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.
In order to make Outlook 2003 connections easier to complete, we changed the mailbox server name to give the appearance of connections to different mailboxes on different servers. Only the AddressBook service understands this changed mailbox server name. Therefore, clients that try to connect directly to Active Directory will fail to make the connection.
However, if many delegate mailboxes are being used, clients that are accessing the Address Book Service will reach a limit on the number of connections any single user can have. This exhausts the maximum number of connections available (20) specified by the default throttling policy that is associated with the user mailbox. In this situation, Outlook 2007 clients and later-version clients do not open multiple additional connections.
The original release of Exchange Server 2010 allows a maximum parameter value for RCAMaxConcurrency of 100. Exchange Server 2010 Service Pack 1 increases the maximum value for RCAMaxConcurrency to 2147483647.
Exchange 2010 Co-Existence Issues
Assume that you configure public folder replication in a mixed Microsoft Exchange Server 2003 and Microsoft Exchange Server 2010 environment. When an Exchange Server 2010 user tries to view an Exchange Server 2003 user’s free/busy information, the user intermittently cannot view the free/busy information, or the user encounters a long delay when he or she tries to view the free/busy information. This is addressed in 2557323.
Exchange 2010 RPC Client Access
This was the #1 support call generator when Exchange 2010 was released:
Outlook connection issues with Exchange 2010 mailboxes because of the RPC encryption requirement - Discusses changes to RPC Client Access encryption requirements between Exchange 2010 RTM and SP1. Should never have been an issue as Outlook should be managed through GPO, right? Well not so much 
On the Topic of GPOs, make sure that they are correctly configured as part of the planning process for deploying Kerberos authentication in Exchange 2010.
Exchange 2003 Updates
An error occurs when an Exchange server 2003 user tries to open more than one delegate mailbox of Exchange Server 2010 in Outlook 2003 - Delegate issue resolved with update for Exchange 2003.
A stub object is left behind in the source database for certain users after a move mailbox operation is complete in Exchange 2003 Service Pack 2 - Logs to Event Log when a stub mailbox is left behind when moved to Exchange 2010. This does not fix the underlying root cause, that is still to be done by the admin, but they now know details about the issue and the monitoring system can alert them to the alert.
Exchange 2010 UDP Notification Support
Folders take a long time to update when an Exchange Server 2010 user uses Outlook 2003 in online mode – Has the details around the changes introduced in Exchange 2010 SP1 RU3.
Note that the article states that *YOU* must manually create a Registry key to enable this feature and then restart RPC Client Access Service to kick the change in.
After you install this update, you have to create a registry subkey to enable the UDP notifications support feature.
Create the following registry subkey to enable the UDP notifications support feature:
HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesMSExchangeRPCParametersSystem
Subkey name: EnablePushNotifications
Type: REG_DWORD
Value: 1
Note If this registry key does not exist, or if its value is set to 0, the UDP notification support feature is not enabled.
“Unknown Error” In Outlook 2003. Work was done to improve the Outlook 2003 online mode experience in Exchange 2010 SP2. The Man (AKA Ross Smith IV) mentions this in the comments section and is also documented in KB 2579172 Items that are deleted or moved still appear in the original folder when you use Office Outlook in online mode to access an Exchange Server 2010 mailbox
“
The UDP notification work we delivered has been working correctly since its release in E2010 SP1 RU3. The underlying issue that many customers have seen with Online Mode clients has been due to view change notification issues; specifically that view change notifications are not returned in the same RPC buffer that included the move/deletion RPC operation response. This issue affected all Outlook versions operating in online mode. In the case of OL2003, this results in extra roundtrips for a client to pull notification information from the server as the original call has completed, so the RPC Client Access service has to fire a UDP notification to get the client’s attention that a change within the folder has occurred.
We have addressed this view change notification issue in E2010 SP2.
“
Outlook 2003 Issues
Concern: Is Having Outlook 2003 Clients Going to Prevent Me from Deploying Exchange 2010? TechNet Wiki page discussing coexistence issues.
Description of the Outlook 2003 hotfix package (Outlook.msp): July 1, 2010 Update to Outlook 2003 to resolve Exchange 2010 coexistence issue in body formatting.
Description of the Office Outlook 2003 hotfix package (Olkintl.msp, Engmui.msp): March 9, 2011 Update to Outlook 2003 to resolve Exchange 2010 coexistence issue where server name changes to a GUID.
Office Outlook 2003 does not connect to two or more additional mailboxes in a mixed Exchange Server 2007 and Exchange Server 2010 environment – Exchange 2007 legacy issue. Resolved in SP2 RU2 for Exchange 2007. This service pack is no longer supported, and all customers must now be on Exchange 2007 SP3.
Update Center for Office, Office servers, and related products central page containing links to latest Office product updates and assistance in installing them Distributing Office 2003 Product Updates
Common Client Access Considerations for Outlook 2003 and Exchange 2010 Exchange team blog with multiple client issues that have to be considered. Some are mentioned above but very worthwhile!
How to configure Outlook to a specific global catalog server or to the closest global catalog server - This is not supported when the mailbox is on Exchange 2010 as NSPI should be on the CAS server’s Address Book Service.
By design Outlook 2003 does not use Autodiscover. Only Outlook 2007 and newer are able to leverage the Autodiscover web service. This should not be a surprise, but like most elephants in the room let’s put it to bed…..
By design Outlook 2003 stores Free Busy information in Public Folders and does not natively use the Exchange Availability web service. Be aware of the replication latency that is inherent in Public Folder replication. This is typically an issue due to room booking conflicts.
Unable to view attachments in OWA 2003, when sent from OWA 2010 – Coexistence issue for Exchange 2003 OWA users. They will see the paper clip icon indicating an attachment is present but will be unable to view the attachment.
Comments
Please also do leave a comment of get in touch with via the “Email Blog Author” in the right hand side of the post if you have items to share or discuss.
Oh, and for the sharp eyed out there who were wondering about the * reference it is here as it did not fit into the flow above.
* To those who remember seeing Police Academy when it was originally released (yes I’m getting old, that was 1984 –eek! ) this was a reference to Commandant Lassard.
Cheers,
Rhoderick
