Exchange 2010 Migration Routing Issue

I am seeing more and more migrations from Exchange 2003 to Exchange 2010 every day. Most migrations are being properly tested to discover issues. One of the most common mail flow issues I come across is messages cannot be delivered between Exchange 2003 and Exchange 2010. There are many potential causes for this to break, but I am going to focus on mail flow from Exchange 2003 fails to deliver messages to Exchange 2010. The reason for this focus is because I have seen many Exchange 2003 configurations that do not follow best practices which results in this failure.

Here is a quick summary of how messages are sent between Exchange 2003 and Exchange 2010. When the first Exchange 2010 hub transport server is installed, the installation wizard prompts for an existing Exchange 2003 server to use as a bridgehead server for a new routing group connector. All Exchange 2010 servers belong to the same routing group and leverage AD sites for routing. Once this first Exchange 2010 hub transport server is in place mail flow between Exchange 2003 and 2010 should work thru the routing group connector. Now let’s review the main cause of why messages do not route properly from Exchange 2003 to Exchange 2010.

More and more organizations are using SMTP gateway devices to protect their internal Exchange environment. They are no longer having Exchange route outbound messages using DNS, but instead use a smart host (edge transport, third party appliance, hosting solution, etc). When the Exchange 2003 server was configured to route mail using the smart host it also broke SMTP communications with Exchange 2010. How? The configuration was done quickly by modify the properties of the Default SMTP Virtual Server for the Exchange server.


Exchange 2003 will attempt to deliver all SMTP messages using the smart host under this configuration, even internal messages where the mailbox is on Exchange 2010. Fortunately there is a simple solution to this dilemma.

The first step is to create a new SMTP connector for the routing group.


On the General tab you need to give the connector a name, enable the connector to “Forward all mail through this connector to the following smart host” using the smart host value from your Default SMTP Virtual Server, and add your Default SMTP Virtual Server as a local bridgehead for your connector.


On the Address Space tab you need to add an SMTP address space of * with an associated cost (the lower the cost the higher the priority for the connector) and select whether this connector should be used for the entire organization or just the routing group it is being created within.


You can review the other tabs and determine if you need to update any other settings. For example the Content Restrictions tab has the maximum message size setting.

Once you create this SMTP connector you must go back into your Default SMTP Virtual Server settings and remove the smart host configuration. Then test outbound mail flow from an Exchange 2003 mailbox to the Internet and to an Exchange 2010 mailbox. After mail flow has been validated you have one more decision to make. Your Exchange 2010 hub transport server must have a Send Connector added before the migration is complete. You could create the new Send Connector and test sending a message to the Internet. When the Send Connector is operational, the SMTP connector in the Exchange 2003 routing group you created can be removed. All Internet mail flow for Exchange 2003 will then route thru the routing group connector and out the Send Connector.