Custom Attributes in Exchange, right click on a mailbox / recipient and take properties, then edit custom attributes as per the screenshots in the tutorial.
if you have alot of users or would have, it will be good to categorize them in an OU, so you can powershell it all those users at once or if you have a few users and can do via Exchange Management Console, then right click on a user's mailbox, properties, on the "General" tab, you would see the "Custom Attributes" button.
http://social.technet.microsoft.com/Forums/en-US/exchangesvradmin/thread/11bdf8d2-9ea3-4c92-b130-d6be8c537b5d Get-Mailbox -OrganizationalUnit "OU=Test,DC=Contoso,DC=com" | Set-Mailbox -CustomAttribute1 "test"
