Kathy in the example above would have her mail software use the name of her mailbox kathy.doe as her POP username. So in that example both Kathy and John would log into the network with network login name johndoe, and John would use the same name as his POP username to tell the mail server he wants the mail in the johndoe mailbox. Kathy would use kathy.doe as her POP username to tell the mail server she wants the mail in the kathy.doe mailbox. Unfortunately, most mail software does not make it easy to use more than one mailbox. The POP3 username and corresponding email address tend to be buried in configuration properties, with an assumption the software will be configured once and rarely or never changed.
Extra mailboxes are most useful when used from different computers.
This is beginning to change, with some newer software enabling multiple email configurations. Mailboxes can be accessed from any network login. In the example above, if Kathy or John had use of network login accounts at work as well as their own account at home, they could access their mailboxes from any account. In principle, mailboxes can be accessed from anywhere on the Internet. However, we enable this only by special arrangement.