The Mail Transfer Protocol (MTP) is an obsolete network protocol used to reliably transfer mail. Request for Comments (RFC) 780, which defined this protocol, allocated Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) port 57 for MTP. This port is no longer officially allocated for this service by IANA.
Replacement
The Simple Mail Transfer Protocol (SMTP), defined in RFC 788 has replaced the Mail Transfer Protocol.
References
Mail Transfer Protocol Wikipedia(Text) CC BY-SA