As mentioned in the previous section and as illustrated in Figure 6-1, the T.37 gateway has the ability to communicate directly with G3 fax devices using the ITU-T T.30 fax protocol, which is covered in detail in Chapter 2. Similarly, the T.37 gateway also communicates natively with the mail server via the SMTP protocol.
SMTP is one of the fundamental elements necessary for understanding and effectively troubleshooting T.37. Therefore, this section discusses some SMTP basics before tackling the T.37 concepts of onramp and offramp.
Connecting through TCP port 25, SMTP is a relatively simple, text-based protocol. Widespread adoption of SMTP started back in the early 1980s, and today SMTP is the de facto standard for e-mail transmissions across the Internet.
SMTP is a "push" protocol, meaning that one device sends or "pushes" an e-mail to another device. SMTP does not allow one to "pull" messages from a server, so e-mail protocols such as Post Office Protocol (POP) or Internet Message Access Protocol (IMAP) must be used in this case.
Because of the "push" nature of SMTP, one side of the transaction is referred to as the SMTP client, and the other side is the SMTP server. The client side is the device that has a mail message to transmit, whereas the server will be receiving the message.
TIP The term mail transfer agent (MTA) is commonly used to signify the SMTP endpoints or servers used to transfer mail from one system to another, whereas the term user agent (UA) is often used to indicate the end-user mail program or client that is used to interface with the mail server. Throughout this section, the term mail server is used interchangeably with MTA, and the term mail client is used interchangeably with UA.
This section covers the commands used in SMTP and basic operation of the protocol. The analysis of the protocol is through showing actual SMTP sessions between a mail server and a mail client. Then, a quick explanation of SMTP concepts relevant to T.37, such as DSN and MDN, is provided.
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