A POP3 Mailbox is the address of where your E-mail is stored before you dial up and collect it.
This will correspond to your E-mail address. Your UKO2 E-mail address will look something like Dad@uko2.co.uk. You could also set up a POP3 Mailbox address for each member of the family. How about Mum@uko2.co.uk?
With UKO2 you can have as many mailboxes as you like. You can have one for each member of the family if you like.