Question regarding mail flow after installing Exchange 2010 and performing swing migration from Exchange 2003 to Exchange 2010. Currently firewall rules point port 143 and 25 to the Exchange 2003 server. At what point do I change the rules to point to the new server? Will the Exchange 2010 be the one handling all internal/external communication? What happens when mailbox is still residing on Exchange 2003 server, will it send mail to Exchange 2010, and then outbound? Will there be any reason to have any firewall rules pointing to the Exchange 2003 server, or can I just replace all the IPs to the new server?