Exchange 2007/2010 to 2010 migration  =  Don’t need to create new SMTP connector in Exchange 2010 – Just add Exchange 2010 as source server and after migration remove old server as source.

Exchange 2003 to 2010 migration = Need to create new SMTP connector on Exchange 2010 server.

You will receive the below error when you try to the add the new Exchange 2010 server to the SMTP connector that already exists in Exchange 2003 (In Exchange 2010 EMC). Exchange 2010 doesn’t recognize the 2003 SMTP connector so a new connector has to be created.