The SMTP protocol (Simple Mail Transfer Protocol) is the Internet standard for distributing e-mail. The SMTP protocol is an older application protocol from the year 1982. It was first described in RFC 821, later, in 2001, an extended version was published under RFC 2821. The SMTP protocol is a client- server protocol, it is text-oriented and is based on the TCP protocol.
The SMTP message consists of the header and the user data. The header contains, among other things, date, reference, recipient, sender, copy recipient, and the user is addressed by a "prompt" for each of these entries. The user data part typically consists of free ASCII text. Messages with multiple recipients on a destination host are transmitted to the destination only once and distributed there.
Transmission procedure of SMTP
Since the SMTP protocol itself cannot retrieve messages, this task is taken over by the POP protocol (POP3) or the IMAP protocol. Emails are entered into the Mail User Agent( MUA), which is the mail client, and delivered to the Mail Transfer Agent( MTA) using the SMTP protocol. The Mail User Agent (MUA) is the SMTP client, and the Mail Transfer Agent is the SMTP server. When the Mail Transfer Agent wants to send the message to a server, it acts as a Mail User Agent, that is, as a client. The SMTP protocol is a client-server protocol that transfers messages from the SMTP client to the SMTP server over a TCP connection. Once the server has received the complete message, it sends an acknowledgement to the client. Message transfer is supported by SMTP commands with a special syntax.
SMTP does not define how a message is relayed from or to a user, nor does it define how received messages are presented or cached by the user. These tasks are performed for the SMTP protocol by other application programs. Compared to X.400, SMTP is a functionally poorer protocol from 1986, specified by the Internet Engineering Task Force( IETF) and first described in RFC 821. RFCs 1891 and 2821 deal with corresponding SMTP extensions, Extended SMTP( ESMTP).