Exchange 2000 – aliasing

I am setting up an Exchange 2000 server,it is one of the domain controllers. The domain is “domain.local”. The Exchange works inoffice, but I know that there is some aliasing needs to be done, I just dont know what or where. The company has an existing domain email of “domaincompany.com”. The inoffice email is “user@doamin.local”, but I know that will not work for internet email.

Any hints or step by steps,
David