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.