210

Chapter 15. Email

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. 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 the session.

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

the server.

POP works best for users who only have one system to read email with since they download their

messages to that machine. 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 10 for more information concerning

Kerberos authentication.

RPOP   POP3 with RPOP authentication, which uses 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, 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 installs POP2 (

ipop2

) and POP3 (

ipop3

) daemons

in the

/usr/sbin/

directory.

15.1.3. SMTP

While the IMAP and POP protocols involve allowing a user to be able to receive 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, on its way toward its final destination. Email servers attempting to move

a message between one another also use 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.

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 served by a