There are lot of useful things can be accomplished using custom attributes in exchange 2007, below is one of the example to configure and test.
Attached document contains step by step creating and configuring custom email address policy based on AD attribute lookup on Exchange 2007. Custom Email Address Policy
The steps involves the below activities:
> Creating Accepted domain to accept email address internally as well as externally.
> Creating custom email address policy with desired domain name.
> Creating a mailbox with the AD attribute value having “Microsoft – Sales” in department.
> Checking the mailbox email address tab to verify the email address was stamped as desired .
In Exchange 2010 there is an option if you want to create email address policy based on Active Directory Organization Units.
Make sure you too test it in your exchange LAB.