Not all of your users will migrate from the old server to the new server at the same time. User accounts on the new server are created and provisioned prior to users' actual migrations. As such, user mailboxes with the same address will reside on both the old and new servers simultaneously during your transition period. Some temporary mail-forwarding rules must therefore be defined to make sure that your users' mail is correctly routed during the transition period.
Even if your organization is implementing new Internet addresses, the old addresses must be maintained on the old server because replies to older messages will be deliverable only if the users' old primary Internet addresses continue to deliver to the correct server. Since all Internet mail for a given domain must go to a single server, specified in an associated MX record, your organization must decide when it will update its MX record to point to the new Sun Java System server.
If the MX record is switched to the Sun Java System server at the beginning of the transition period, you must configure the Sun Java System server so that any mail it cannot deliver to a local mailbox will be sent to the corresponding mailbox on the old server. Moreover, as new users are provisioned on the Sun Java System server, you must define forwarding rules on the new server so that any mail that would normally be delivered to the new mailboxes will instead be forwarded to the corresponding user mailboxes on the old server. As each user migrates to the new server, you must then delete the first forwarding rule on the new server, and define a new rule on the old server to forward all of the user's mail to the corresponding Sun Java System mailbox.
On the other hand, if the MX record will point to the old server until the end of the transition phase, you must configure the old server to send mail it cannot deliver locally to the corresponding user mailboxes on the Sun Java System server. As users migrate to the new server you must then define new rules on the old server to forward mail sent by other users on the old server to the users' new accounts on the Sun Java System server.