Red Hat LINUX 7.2 Reference Manual page 261

Section 16.1:Protocols
• KPOP — POP3 with Kerberos authentication. See Chapter 8, Using Kerberos 5 on Red Hat Linux
for more information concerning Kerberos authentication.
• RPOP — POP3 with RPOP authentication, which utilizes an ID issued per user, similar to a
password, to authenticate POP requests. However, this ID is not encrypted, so RPOP is no more
secure than standard POP.
Many POP servers, clients, and assorted other applications are available with Red Hat Linux. If you
prefer a graphical email client, Mozilla Mail is an excellent choice. In addition, other email utilities,
such as Fetchmail, can retrieve email via POP. If you are using your Red Hat Linux system as a mail
server, the imap package contains POP2 (ipop2) and POP3 (ipop3) daemons in the /usr/sbin
directory.
16.1.3 SMTP
While the IMAP and POP protocols involve allowing a user to be able to receive and read their email,
the Simple Mail Transfer Protocol (SMTP) is used to send email. Outgoing email uses SMTP to
move from the client's machine to the server, where it moves along toward its final destination. Or,
two email servers attempting to move a message between one another utilize SMTP so they can com-
municate, even if they are totally different platforms.
SMTP uses port 25 on the server for its communication. A basic SMTP exchange begins with the
connecting system issuing a MAIL From:
The receiving system responds with a 250 message to acknowledge receipt of the first command.
Then, the connecting system hands the email addresses to receive the message to the receiving system,
followed by a DATA message. This tells the receiving system that the next part of the communication
will be the actual body of the email message. When the connecting system is finished with the email
message, it places a single dot (.) on a line. At that point, the message is considered sent.
SMTP also handles cases where email needs to be forwarded between systems, when the receiving
system knows where to send the message. The protocol can verify that certain users are indeed served
by a particular mail server (the VRFY command) or expand a mailing list (the EXPN command). Email
can also be relayed between two SMTP servers, if both systems permit such activity.
Unlike IMAP and POP, SMTP does not require authentication in its most basic form. This has made
possible a lot of spam, due to the fact that a non-local user could use your system to send or relay
mail to entire lists of recipients, using your system's resources and bandwidth to deliver the junk mail.
Modern SMTP applications have gone to great length to minimize this behavior by restricting relaying
and allowing only known hosts to send email.
RFC-821 outlines the basic behavior of SMTP, but several SMTP extensions, made possible by RFC-
1869, have added additional functionality to SMTP over the years by making new commands avail-
able. By initiating a conversation with an SMTP server with an EHLO command rather than HELO,
the connecting server can identify itself as one that supports SMTP extensions. The receiving server
< email-address > command to initiate exchange.
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