Simple Mail Transfer Protocol (SMTP) is a protocol designed to transfer electronic mail reliably and efficiently. SMTP is a mail service modeled on the FTP file transfer service. SMTP transfers mail messages between systems and provides notification regarding incoming mail.
SMTP is independent of the particular transmission subsystem and requires only a reliable ordered data stream channel. An important feature of SMTP is its capability to transport mail across networks, usually referred to as "SMTP mail relaying". A network consists of the mutually-TCP-accessible hosts on the public Internet, the mutually-TCP-accessible hosts on a firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN environment utilizing a non-TCP transport-level protocol. Using SMTP, a process can transfer mail to another process on the same network or to some other network via a relay or gateway process accessible to both networks.
Protocol Structure
SMTP commands are ASCII messages sent between SMTP hosts. Possible commands are as follows:
| Command | Description |
| DATA | Begins message composition. |
| EXPN <string> | Returns names on the specified mail list. |
| HELO <domain> | Returns identity of mail server. |
| HELP <command> | Returns information on the specified command. |
| MAIL FROM <host> | Initiates a mail session from host. |
| NOOP | Causes no action, except acknowledgement from server. |
| QUIT | Terminates the mail session. |
| RCPT TO <user> | Designates who receives mail. |
| RSET | Resets mail connection. |
| SAML FROM <host> | Sends mail to user terminal and mailbox. |
| SEND FROM <host> | Sends mail to user terminal. |
| SOML FROM <host> | Sends mail to user terminal or mailbox. |
| TURN | Switches role of receiver and sender. |
| VRFY <user> | Verifies the identity of a user. |
Related Terms: POP3, POP, IMAP4, TCP, FTP
Sponsor Source: SMTP is defined by IETF (http://www.ietf.org) RFC 2821.
Reference: http://www.javvin.com/protocol/rfc2821.pdf : Simple Mail Transfer Protocol