We are getting ready to migrate the Client Access and Transport roles from Exchange 2003 to 2010. This will be a coexistent scenario. We have numerous hosts that are currently using Exchange 2003 as an SMTP relay. We will be attempting to point most of these hosts to the new Exchange 2010 during the migration, but is it still possible for SMTP traffic to still relay through the 2003 server after the Transport role has been moved to the the new Exchange server? We realize we will not be able to access a couple of these hosts during the migration window and we were trying to determine if mail would halt from those hosts.

As you can't remove the functionality from Exchange 2003, nothing will change. If you configure the platform to send email out via Exchange 2010, then that will happen for those relayed messages. Once Exchange has accepted the message, it is routed like any other email. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources | In the UK? Hire Me.

Thanks Simon. Just want to get a little clarification. As I understand it, all email will be sent out via Exchange 2010 Hub Transport. Does this mean that hosts still pointing to 2003 will send SMTP traffic to Exchange 2003 and that will then pass the message off to Exchange 2010 which will then send it out to the recipient? Do any additional connectors need to be in place for this transfer besides the default connector between Exchange 2003 and 2010? Thanks

No additional configuration will be required. If internal email is flowing correctly between the servers, then email from a relaying machine will do so as well. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources | In the UK? Hire Me.

Thank you, Simon. Sounds like we're good.