migration exchange 2000

I have a seperate domain created with Active Directory and Exchange 2000. I also have the existing domain, with exchange5.5, which is aour production server. What I would like to do,is be able to migrate any mail box and all it’s parts to the new exchange server and when someone sends mail to my old email address, it will come to my new mail address on the new exchange server. I have added the 2 new servers to the existing domain on the same wire and I can see the other servers and ping and get a reply, yet when I send mail from my new 2000 exchange server to the 5.5 server I get an error that the host could not be found. What do I need to do to be able to make this happen.