Exchange server on SBS2003

Hi
We have a SBS 2003 running with 5 users (20 users available). The server is running Active directory and exchange server. I have been assigned to somehow create 5 new mailboxes wich will be “attached” to another domain. In other words i need to be able to send and receive mail from another doamin than our own. Is this possible? Is this what would be a “hosted exchange server?”. We have a domain with 5 exchange users and now need to “add” 5 more but from another domain. I hope I’m not to fuzzy in the explanation, I’m not so hype on exchange server and wonder if you can acomplish this? Any ideas, suggestion or guidelines would come in handy and be much appreciated. THANX!