If mailboxes are not showing up under a user's properties you will need to add that user to a specific domain.



For example, if your domain is called qbik.com and your user is Frank, to see frank@qbik.com under the email tab you will need to either:

Edit the domain qbik.com from the email properties
In the address tab specifically add the email address handler Frank and deliver to the user Frank



Or:

Check the box enable reception of mail for addresses where no matching address handler exists