Well, after almost 14 months of deploying Exchange 2010 and tweaking the Outlook 2003 settings via GPO’s to give users an acceptable experience Microsoft adds support for User Datagram Protocol (UDP) notification functionality back into Microsoft Exchange Server 2010. By doing so they recognize that a lot of businesses & organizations will be using Outlook 2003 for a while and that not all of them where happy to deal with the way Outlook 2003 functions with Exchange 2010. More information on the UDP issue can be found here http://support.microsoft.com/kb/2009942 (In Outlook 2003, e-mail messages take a long time to send and receive when you use an Exchange 2010 mailbox). Now most my customers use cached mode where possible and a GPO Setting to reduce the Maximum Polling Frequency registry entry to 5 seconds helped. But there are places where cached mode is not an option (Terminal Services) or people don’t accept this change in behavior and go with Outlook 2007 instead of 2010 or even choose to deploy Exchange 2007 over 2010. All because of this dropping of the UDP notification support.
Now this functionality will be back with in Exchange Server 2010 Service Pack 1 Roll-Up 3 (SP1 RU3). Good news for people dealing with Outlook 2003 and Exchange 2010. Less good news for the people dealing with the GUI bug that Exchange 2010 SP1 introduced where the Exchange Management Console does not show all database copies after upgrading to Exchange 2010 SP1. This is set to be fixed in Roll-Up 3 but to get the UDP support back they adjusted the release schedule for the E2K10 Sp1 Roll-Up 3, which is now expect to be released in March 2011. So we’ll have to wait a bit longer for that fix. As you noted you need to be running Exchange 2010 SP1 to get this backward compatibility support for outlook 2003.
Read this announcement on the Exchange Team Blog: UDP Notification Support Re-added to Exchange 2010