Every domain has a postmaster login.
This is the Master Account of the entire domain and only postmaster can perform administrative tasks regarding mail behaviour for this account. After you have successfully logged in as postmaster, you can add and delete POP users. Each domain has its own set of POP users so a POP account for one domain is un-related to a POP account of another domain even if they have the same login. You cannot delete the postmaster account. You can designate any one of the POP accounts as the CatchAll account by clicking on the CatchAll button associated with that account.