POP is actually a protocol for checking email, but the same username and password is usually used for sending.
With either ListMail send method, SMTP or Internal PHP mail() , email is queued to your (web host's) outgoing server for sending. SMTP is highly recommended as it's much faster and also much more reliable. Depending on your host, you may or may not have to enter authentication information to be able to send.
When sending, a return-path (used for bounces/undeliverables) is set for your message. This address is set in ListMail's config menu, only appears in the headers of the message, and is not normally shown to the user when they read your emails.
The "Send From Name" and "Send From Email", configurable on a per-list basis on the List Settings page, are what your subscriber will see and will be used for replies to your messages. These variables do not necessarily need to match an existing mailbox on your host.
