Then they have one mailbox for John and one for Kathy. However, Kathy must still log onto the network with network login name jsmith . To access her mailbox, she has her mail software connect to the mail server with POP username ksmith . John uses jsmith for both his network login and mail login. This is primarily of interest to people with their own domains, who can have addresses in their domains mapped to mailboxes with unrelated names.
All domains have a default mailbox.