How to communicate two exchange 2000 servers

I have two domains which are not at the same Active Directory Forest. Each one has his own Exchange Server and due to server obsolescense and because we are putting in production a new domain we need to take out of production one of these ExServer, but while this momment occurs, we need to redirect the messages from the old ExServer to the new own.

Which are the basic steps to take into account to get this?. Is there any Receipt?

Thanks a lot in advance,

Dewey