The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)

SMTP Overview, History and Standards 1 2 3 SMTP Connection and Session Establishment and Termination

Modern E-Mail Communication Using DNS and Direct Delivery

The creation of DNS radically changed how e-mail delivery worked. DNS includes support for a special mail exchanger (MX) record that allows easy mapping from the domain name in an e-mail address to the IP address of the SMTP server that handles mail for that domain. I explain this in the topic on the regular e-mail address format, as well as the dedicated topic on DNS e-mail support.

In the new system, SMTP communication is much simpler and more direct. The sending SMTP server uses DNS to find the MX record of the domain to which the e-mail is addressed. This gives the sender the DNS name of the recipient's SMTP server. This is resolved to an IP address, and a connection can be made directly from the sender's SMTP server to the recipient's to deliver the e-mail. While SMTP still supports relaying, direct e-mail delivery using MX records is faster and more efficient, and RFC 2821 makes clear that this is now the preferred method.

In this new system, SMTP is generally only used for two transfers: first, from the sender's client machine to the sender's local SMTP server, and then from that server to the recipient's local SMTP server, as shown in Figure 301. (A distinct mail access protocol or method is used by the recipient for the last leg of the journey.) Each transfer of an e-mail message between SMTP servers involves the establishment of a TCP connection and then the transfer of the e-mail headers and body using the SMTP mail transfer process. The next two topics describe in detail how this occurs.

SMTP Overview, History and Standards 1 2 3 SMTP Connection and Session Establishment and Termination