Simple Mail Transfer Protocol Port. Smtp is basically a set of commands that authenticates and directs the transfer of email. Typically user level mail clients make use of smtp only for sending messages to a mail server for relaying.
Smtp usually is implemented to operate over internet port 25. It is a program used for sending messages to other computer users based on e mail addresses. Esmtp is a new version of smtp with added functionality.
Tcp port 57 is assigned to mail transfer protocol mtp.
The smtp server is an always on listening mode. Udp port 69 is used by trivial ftp tftp. An alternative to smtp that is widely used in europe is x400x500. As soon as it listens for a tcp connection from any client the smtp process initiates a connection through port 25.