When multiple email addresses have been configured for a user, only one email address can be set as the primary/Reply To address. The primary address is the From address when sending a message. The user cannot select an alternate From address from their list of addresses.
You can buy commercial software that enables this functionality (http://www.ivasoft.biz/choosefrom2007.shtml). However, for the cheap (ahem, cost sensitive) among us there is a workaround.
- For any secondary address you want to assign to a user, create a distribution group with that address.
- Make the user the only member of that group.
- Assign the user Send As permission for the group.