SMTP: Definition of SMTP

Simple Mail Transfer Protocol (SMTP) is designed for reliable and efficient mail transfer. It is widely used in government and education facilities and is also the standard used by the Internet for mail transfer.

The Simple Mail Transfer Protocol would be an "application layer" protocol when slotted into the OSI model. It is an open protocol because the only assumption it makes of the transport mechanism below it is that it will provide a reliable service.

In most cases, SMTP is used in conjunction with the Transmission Control Protocol (TCP) service, which provides the reliable transport layer (Service). Other transport mechanisms that are mentioned as supported in the specification are the Network Control Program (NCP) service, the Network Independent Transport Service (NITS), and the X.25 service.

Requests for comment (RFCs) provide specifications for the protocol:

• RFC-821 specifies the exchange protocol and is the basis for SMTP.
• RFC-822 specifies the message format.

The internet mail service is based on these RFCs.

RFC 821 for SMTP

RFC-822 for the message format

For more information, query on the following words in the Microsoft Knowledge Base:

Transmission Control Protocol/Internet Protocol