Free POP Resources
The Post Office Protocol
An article that explains how POP works.
Introductions to POP1 and POP3.
Email Protocols : SMTP, MIME, POP and IMAP
These are presentation slides including information about smtp, and the other email protocols
Eudora Email Primer (pdf link)
"Section Two, POP, IMAP, and Real World Issues, discusses the two most popular mail access protocols today, POP and IMAP, and how they implement mail access modes to provide real-world solutions for today?s formidable email challenges."
POP3 RFC document.