I have a dilemma that I'm hoping to have some of you more seasoned folks shed some light. I have 1 Exchange 2003 server and 1 Windows 2003 DC in domainA.local.com. Now my goal is to migrate about 25 users in domainA.local.com to a totally "new" Windows 2008 R2 with Exchange 2010, which is also in domainA.local.com. I did not want the old y AD in the new AD forest, so I created from scratch. In essence I have duplicated the 2003 to 2008. My challenge is I need to start moving users to the Exchange 2010 domain a few users at a time, which I want to test slowly. Which will require me to remove userA from the old domain and add them to new domain. I have an MX record with priority 10 for the 2003 server and priority 20 for the 2010 server. I ran a test, both ports open on firewall. I created a user on the exchange 2003 host called userX.domain.com, and created another user on Exchange 2010 called userY.domain.com. I send a message to userY.domain.com, If I do that mail never gets delivered to userY(20) as it sees smtp running on 10(I believe), and returns the message as “address couldn’t be found”. The problem is I need to have both mail servers running and accepting email from internet. So how can I have both mail servers accepting email that if I email comes in a does not find a recipient on the first priority server it attempts to deliver on priority 20 server? I guess I do not fully understand how this process is supposed to work. Any info that I can be directed to or replied to here would be greatly appreciated. I have been trying to figure this out for a month now.