The SMTP Model

The exchange of E-Mail using TCP/IP is performed by a message transfer agent (MTA). Users normally don't deal with the MTA. The system administrator is responsible to set up the local MTA. The SMTP protocol describes how two MTAs communicate with each other using a single TCP connection.

The SMTP standard is one of the most widely used upper layer protocols in the Internet Protocol stack. As its name implies, it is a protocol that defines how to transmit messages (E-Mail) between two users.

SMTP uses the concept of spooling. The idea of spooling is to allow E-Mail to be sent from a local application to the SMTP application, which stores the E-Mail in some device or memory. Once the E-Mail has arrived at the spool, it has been queued. A server checks to see if any messages are available and then attempts to deliver them. If the user is not available for delivery, the server may try later. Eventually, if the E-Mail cannot be delivered, it will be discarded or perhaps returned to the sender. This is known as an end-to-end delivery system, because the server is attempting to contact the destination to deliver, and it will keep the E-Mail in the spool for a period of time until it has been delivered.

SMTP is found in two RFCs:
RFC 822 describes the structure for the message, which includes the envelope as well. RFC 821 specifies the protocol that controls the exchange of E-Mail between two machines. The next figure illustrates a general model of SMTP: