Hi Scott and thanks for your reply. Even so, I didn't get it.
Let me explain it all:
The Client actually has a site hosted in another hosting company and a in-house Exchange Server. The whatever.pt domain is registered through an official register (as required for .pt domains) and the dns are pointed to the actual hosting company. So, I also assume that they're re-routing the e-mail from the hosting company to the Exchange Server.
Now, they want me to create a new website hosted here, in dreamhost. I'll contact the official register for .pt domains and ask for the change in dns records so that the whatever.pt url point to my dreamhost installation. But if I only do that, the e-mail sent to @whatever.pt will be lost right? So, imho, what I do need to do is, create a custom dns (MX record) here in dreamhost under the whatever.pt domain where the value will be the IP from the actual Exchange server so that the e-mail sent to @whatever.pt will then be re-routed from dreamhost to the Exchange Server. Is this right? If it is, I still have a problem. I don't know how to do it.