Postmaster e-mail account (mailbox) is created automatically during the setup process. The postmaster account is required by regulations (RFC 5321) to be present on every mail domain.
On a contrary to catch-all mailbox that is disabled on our shared mail servers due to the security reasons, the postmaster is a technical mailbox that cannot be removed. By default, the postmaster password will be the same as the password you set when you created your account.
Postmaster account can be used to manage all mail server accounts through My Services web interface. Read our knowledge base article on How Can I Manage All POP/IMAP Mailboxes on POP Mail Server Using My Services? for more details.
Note: There is no way to disable delivery or enable Auto-Reply/Forwarding options for the postmaster mailbox.