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

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Chapter 240
Simple Mail Transfer Protocol (SMTP)

Simple Mail Transfer Protocol (SMTP)

•  Today known as Electronic Mail, or email.
•  RFCs 821, 822, 974.
•  Email still cannot transport packages and other items.
•  Email is very fast and guarantees delivery.
•  Three protocols are used for today’s email:
•  SMTP—operates over TCP
•  POP—operates over TCP
•  DNS—operates over UDP
•  SMTP allows for the sending/receiving of email.
•  POP allows us to intermittently retrieve email.
•  DNS makes it simple.

RFC 822 defines the structure for the message, and RFC 821 specifies the protocol that is used to exchange the mail between two network stations. It truly is amazing how old the original mail protocol is, and it is still in use today.

So we have email to send to one another, completely bypassing the postal system. There are some who call the postal system “snail mail.” True, if stated without emotion; otherwise, I call these people who state that arrogant. Many people today still immensely enjoy receiving a handwritten letter from a family member, friend, or a business correspondence through the postal system. Suffice it to say that the postal system will be here for many years to come.

Also, I have the hardest time sending packages through the email system and some I do get through (attachments) get banged up along the way. Yes, the postal system is old and cranky, but it works, and in some cases better than email. But this is an electronic discussion and I will keep it at that. Email does have many, many advantages and one of the top advantages is speed. The biggest disadvantage: lack of emotion. Like everything else, email has its place, but it is not 100 percent of the pie; it is merely another form of communication.

In order to send and receive mail between users, there are actually two protocols (possibly three) that are used:

SMTP: Used for the actual transport of mail between two entities (mail servers).
POP (Post Office Protocol): A protocol that allows single users to collect their mail on one server.
DNS: Used to identify the mail hosts for a domain or hostname.

Mail can be sent and received using only SMTP, but the other protocol involvement makes it much easier to use and is more efficient. This section will concentrate on SMTP and POP. The hooks into DNS were already explained in the Mail Exchanger section of DNS and are shown again on slide 264. SMTP was created first and then POP, so I will start with the SMTP protocol. This is a protocol that allows users to transmit messages (mail) between other users. It is one of the most widely used applications of the TCP/IP protocol.


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