Mail Services

Electronic mail is an important network service because it is used for interpersonal communications. Some applications are newer and fancier. Other applications consume more network bandwidth. Others are more important for the continued operation of the network. But email is the application people use to communicate with each other. It isn’t very fancy, but it’s vital.

TCP/IP provides a reliable, flexible email system built on a few basic protocols. These are: Simple Mail Transfer Protocol (SMTP), Post Office Protocol (POP), and Multipurpose Internet Mail Extensions (MIME). There are other TCP/IP mail protocols. However, these are the basic mail protocols. Our coverage concentrates on the three protocols you are most likely to use building your network: SMTP, POP, and MIME. We start with SMTP, the foundation of all TCP/IP email systems.

Simple Mail Transfer Protocol

SMTP is the TCP/IP mail delivery protocol. It moves mail across the Internet and across your local network. SMTP is defined in RFC 821, A Simple Mail Transfer Protocol. It runs over the reliable, connection-oriented service provided by Transmission Control Protocol (TCP), and it uses well-known port number 25.[19]

Table 3-1 lists some of the simple, human-readable commands used by SMTP.

Table 3-1. SMTP Commands





HELO <sending-host>

Identify sending SMTP


MAIL FROM:<from-address>

Sender address


RCPT TO:<to-address>

Recipient address



Begin a message

Reset ...

Get Windows NT TCP/IP Network Administration now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.