Domain Postmaster
If a user sends an e-mail to a non-existent account it will be forwarded to this user. It's an administrative address that receieves all undeliverable mail.

This can be either a user on your machine or a normal e-mail address. If you want it to be a user on your machine enter the username or use the user chooser by clicking on the "..." button.

This field is necessary if you have entered a domain to recieve mail for. You can always change the postmaster in the "Mail Server Management" portion of the "System Management" module at a later time.