Combining domains in Exchange 5.5

Hi everyone

I have been given the task of amalgamating two domains together in Exchange 5.5. Both companies have merged and now want to use the same email system. They have two different domain names and have a 128k link. Can anyone please advise as I have to go and advise this person how to go around it and to be honest, I am pretty new to this and would appreciate any help.

Thank you very much

Debbie