If you have an email mailbox called firstname.lastname@example.org which you login to using the username fred123 and password 123fred and use the POP3 server mail.btinternet.com you use the following details:
Type: if the Mailserver specified is a POP3, select POP3. If it's an IMAP server, select IMAP.
This creates a new entry called email@example.com (this is not your actual email address it is just Username@Hostname), which when you click it will ask for your password.
Username is what you login with to get to your mailbox and not necessarily your mailbox name.
Hostname is the domain part of your email address (the bit after the @)
Mailserver is just that the POP3/IMAP server.
Note: there was a problem with Usernames containing @ (i.e. you actually login with a username like firstname.lastname@example.org) which did not work. I'm not sure if this has been fixed yet.