Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 241
SMTP Functions

The protocol is relatively simple. A message will be created, properly addressed, and sent from a local application to the SMTP application, which will store the message. The server will then check (at periodic intervals) to see if there are any messages to deliver. If there are, the mail server will try to deliver the message. If the intended recipient is not available at the time of delivery, the mail server will try again later. The mail server will try a few times to deliver the message and, if it cannot, will either delete the message or return it to the sender.

The address has the general format of local-part@domain-name. By this, you should recognize the domain name format. For example, an address at the SMTP header could be matt@engineering.naugle.com. This would indicate that the message is addressed to a user named Matt in the domain of engineering.naugle.com. When DNS is used to look up the mail handler for Matt, it will have some sort of entry like:

From this, the name will be looked up and the mail will be delivered to that host.

There are two entities to this system, the sender SMTP and the receiver SMTP, that are used to transport mail between two systems. The sender SMTP will establish communications with a receiver SMTP. Attachments are allowed with Internet email but not directly with the protocol used in SMTP (sendmail protocol). The Internet email mailer program

SMTP (or more specific, sendmail) can only handle text. Therefore, most email applications convert an attachment to text before sending. A common type is MIME (Multipurpose Internet Mail Extensions, beyond the scope of this book). At the receiver, the email application converts the attachment back to its original format.