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202 Chapter 16. Email

16.1.2. POP

The Post Ofﬁce Protocol (POP) allows email clients to pull off email from remote servers and save

those messages on their local machine. Most POP email clients are automatically conﬁgured to delete

the message on the email server after it has been successfully transferred to the client’s system, though

this can usually be changed.

To connect to a POP server, the email client opens a TCP connection to port 110 on the server.

At the time the connection is made, the POP server sends the POP client a greeting, after which

the two machines send each other commands and responses speciﬁed in the protocol. As part of this

communication, the POP client is asked to authenticate itself in what is called the Authentication State,

where the user’s username and password are sent to the POP server. If authentication is successful,

then the POP client moves on to the Transaction State, where commands like LIST, RETR, and DELE

can be used to list, download, and delete the messages from the server, respectively. Messages set to

be deleted are not actually removed from the server until the POP client sends the QUIT command

to end the session. At this point, the POP server enters the Update State, where it deletes the ﬂagged

messages and cleans up any resources remaining from this session.

POP is a much simpler protocol than IMAP, since fewer commands are sent between the client and the

server. POP is also slightly more popular, although most major email clients can use either protocol.

Most POP users only have one system that they use to read email, and they download their messages

to that machine for storage. POP also works well if you do not have a constant connection to the

Internet or the network containing your mail server.

Several RFCs cover the POP protocol, but RFC-1939 deﬁnes the basic outline of POP3, the current

version.

Occasionally, you may run into lesser-used POP protocol variants:

• APOP — POP3 with MDS authentication, where an encoded hash of your password is sent from

the email client to the server rather then sending the password in plain text.

• KPOP — POP3 with Kerberos authentication. See Chapter 11 for more information concerning

Kerberos authentication.

• RPOP — POP3 with RPOP authentication, which utilizes an ID issued per user, similar to a pass-

word, 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, either Mozilla Mail or Ximian Evolution is a good 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. Two email

servers attempting to move a message between one another also utilize SMTP to communicate.

SMTP uses port 25 on the server for communication. A basic SMTP exchange begins with the con-

necting system issuing a MAIL From:

email-address command to initiate exchange. The re-

ceiving 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.

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Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 202

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