SMTP(Simple Mail Transfer Protocol)

The actual mail transfer is done through message transfer agents. To send mail, a system must have the client MTA, and to receive mail, a system must have a server MTA. The formal protocol that defines the MTA client and server in the Internet is called the Simple Mail Transfer Protocol (SMTP).

SMTP simply defines how commands and responses must be sent back and forth. Each network is free to choose a software package for implementation. SMTP uses commands and responses to transfer messages between an MTA client and an MTA server. Each command or reply is terminated by a two-character (carriage return and line feed) end-of-line token. Commands are sent from the client to the server.