Outlook 2003 (which is the bundle package with SBS 2003) uses the single login account that is associated with your active domain user profile.
You can set rules in exchange that allow
address@domain1.com,
address2@domain1.com or even
address3@domain3.co.uk to associate those emails with individual users accounts, or alternatively with distribution lists.
If your users have allocated "send on behalf" permissions for their accounts you can reply to emails adding the "from" field so that the alternative email will appear.
If you need to send on behalf of a distribution group then things become really fun. Haven't got the details to hand now. I know I had to do a serious bit of
to find a solution.