Hi,
You can forward a copy of your test users mail from inside the user details of server-manager - choose Email Delivery to be Both deliver locally and forward.
If you want your new mail server to test the same domain name this won't work. Most of your options would involve delegating the new server as the mail server. I would make the new mail server a variant of your domain - ie main domain = mydomain.global; test domain = mydomain.local. This would work for testing processing of mail on the new server, but wouldn't test the ability of your new mail server to pick up your original mail. Also if you want to test spam filtering (for example) you may need to disable this first in the original server to get a clear idea. Maybe if you gave some more details - ie other server, network setup and what facets of the new mail server you are trying to test you might get some fuller ideas.
Delegation of the domain mail can be easily achieved from the server manager - but this would not leave an active copy for the rest of the network.