MX won't work. MX records provide a mechanism for delivery if a server is unavailable. If both are available it'll send to the server with the lowest metric not both.
Given you are performing a slow migration this is what I suggest you do:
1. On the Exchange 2000 server create mailboxes for all your users.
2. On the Exchange 5.5 server create an additional e-mail address for each user (name@servername.localdomainname). So they can recieve e-mail on that address.
3. On the Exchange 2000 server create custom recipients for each user forwarding e-mail to the 5.5 server.
4. Change the MX record to point to the Exchange 2000 server. All 5.5 users will recieve e-mail via the forwarding.
5. As you move users onto the 2000 system simply delete the forwarder.
Optional bit:
If the e-mail flow must be bi-directional (5.5 users sending to 2000 users and vice versa) then:
6. On the Exchange 2000 server create an additional e-mail address for each user (name@servername.localdomainname). So they can recieve e-mail on that address.
3. On the Exchange 5.5 server create custom recipients for each user forwarding e-mail to the 2000 server.
Then as you move each user delete the Exchange 2000 custom recipient and activate forwarding on the Exchange 5.5 server.
Creation of all these custom recipients can be avoided if the migration is done over a very short space of time.
