Receiving mail for multiple domains is simply done in the Global Domain document. Put your primary domain in there, and then add any secondary domains into the "Alternate Internet domain aliases" field.
As for Web sites, in D6 all you have to do is go to the Configuration -- Internet Sites view and create a new Internet Site document. In that document you set everything you need, such as a home URL, directory, Java info, Web engine, security, etc. The administration help database has a lot of information on setting up Web site documents, as well.
This was first published in August 2004