When performing a mail migration from Exchange2003/2007 to Exchange 2010 a little understanding of how proxy and redirection work can smooth your migration woes. A quick definition of proxy and redirection can help eliminate confusion between the two. A redirection happens when a device is configured to communicate with Exchange at specific URL and the initial server sends the device a redirection to a different URL that is closer or better suited to handle the device. The device then updates it configuration to talk to the new location. A proxy happens when a device is configured to a specific URL and the responding server gets the required data from mailbox server on behalf of the device regardless of version and or location. To help further explain the situation the following graphics can be used to as an example of a proxy and a redirect.
Figure 1. Redirection
Figure 2. Proxy
One could look at the above graphics and could make the determination to always proxy due to the belief that it is faster and would appear to be simpler. However, redirection is a very effective way of moving devices to servers more suited to handle the request. For Example, if a user from the UK has a device configured to communicate with a server in the USA, it is along away across the pond for a proxy request from server to server. In this particular scenario a redirection would be better suited to allow the device to talk to a server in the UK that is closer in proximity.
Proxy and Redirection determination is handled by the existence of the external URL on the 2007/2010 Client Access Server. If all Internet facing servers have their External URLs configured then those servers will always redirect devices to the Client Access Server closest to the mailbox. If only 1 site is Internet facing, that one site will Proxy connections to all other exchange Servers in the environment.
When performing a migration from 2003 to 2010 it is always best practice to point the external address at a 2010 Client Access Server. Since Exchange 2003 has no concept of sites and Internet connected sites, 2010 will always proxy to 2003. The last trick to the configuration is to ensure that devices configured to point to an Exchange 2010 server and have mailboxes on Exchange 2003, need to have integrated authentication enabled on the Exchange 2003 ActiveSync Virtual Directory. Unfortunately, when you enable integrated authentication on 2003 it goes behind you and resets the configuration back to basic every so many minutes. The Fix for this issue can be located here at the following MS support article.
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