Post Office Protocol (POP) Question:

Irrespective of the mail retrieval protocol, email clients typically use the Message Submission flavor of the Simple Mail Transfer Protocol (SMTP) to send messages. POP3 contains a protocol extension, known as "XTND XMIT", that allows clients to transmit outbound mail. The Qualcomm qpopper and CommuniGate Pro servers and Eudora clients are examples of systems that optionally utilize the XTND XMIT methods of authenticated client to server email transmission.