One of our customers needed a way to use multiple mail domains. He already had a SBS2011, The Exchange Server on it was configured for the primary E-Mail Domain of the Company. The E-Mails from this Domain were fetched with the POP3 connector and all outgoing Mail was sent to the smart host of the E-Mail Provider.
The additional E-Mail Domains required a similar setup.
Unfortunately Exchange does not allow you to change the smart host for outgoing E-Mail based on the sender. There are third party Exchange addons, that do provide this ability, but I preferred to solve the problem with a Linux Mail Server. The configuration I will describe to you is fairly basic and the scenario it is used in is somewhat special. But the Article will still give you a fairly good impression on how to get a basic Mail Server running and how all the different pieces work together.