The postmaster account for your domain is the
'Admin' account, and is used to manage your POP3 users and other email
features. When you log into the Email Admin Console as the postmaster,
you have full permissions to create new POP3 accounts, change passwords,
setup mailing lists, and more.
Also, email sent to your domain that cannot be sent to an actual user
account or forwarding address usually ends up in this account, as well
as reports of unusual activity, abuse and other important events. You
(or your designated postmaster) should regularly check this account for
missed messages or other mail domain related events.
You access your postmaster account like any other account, using the full email name, along with your password, to login.