It’s taken me too long to find this.
(For the uninitiated, Exchange Recipient Policies tell Active Directory how to automatically generate email addresses for your users.)
If you want to tell a recipient policy in Exchange 2003 (and presumably 2007) how to generate the localpart other then just username, use these expanos:
%s : Surname (last name)
%g : Given name (first name)
%i : Middle initial
%d : Display name
%m : Exchange 2003 alias
%rxy : Replace all subsequent characters x with character y in username. If x = y, the character will be deleted.
There are other things you can do like chopping the string to a certain number of characters, see the full article for details.
No longer will I have to manually add first.last@domain.com to every user I create :D
http://support.microsoft.com/Default.aspx?kbid=822447