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.At this point, thePOP server enters the Update State, where it deletes the flagged messages and cleans up anyresources remaining from this session.POP is a much simpler protocol than IMAP, since fewer commands are sent between theclient and the server.POP is also slightly more popular, although most major email clientscan use either protocol.Most POP users only have one system that they use to read email, and they download theirmessages to that machine for storage.POP also works well if you do not have a constantconnection 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, thecurrent version.Occasionally, you may run into lesser-used POP protocol variants:" APOP POP3 with MDS authentication, where an encoded hash of your password issent 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 con-cerning Kerberos authentication." RPOP POP3 with RPOP authentication, which utilizes an ID issued per user, similar toa password, to authenticate POP requests.However, this ID is not encrypted, so RPOP isno 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 goodchoice.In addition, other email utilities, such as Fetchmail, can retrieve email via POP.Ifyou are using your Red Hat Linux system as a mail server, theimappackage contains POP2(ipop2) and POP3 (ipop3) daemons in the/usr/sbin/directory.16.1.3.SMTPWhile the IMAP and POP protocols involve allowing a user to be able to receive and readtheir email, the Simple Mail Transfer Protocol (SMTP) is used to send email.Outgoing emailuses SMTP to move from the client s machine to the server, where it moves along towardits final destination.Two email servers attempting to move a message between one anotheralso utilize SMTP to communicate.SMTP uses port 25 on the server for communication.A basic SMTP exchange begins withthe connecting system issuing a MAIL From: email-address command to initiate ex-change.The receiving system responds with a 250 message to acknowledge receipt of the Chapter 16.Email 209first command.Then, the connecting system hands the email addresses to receive the mes-sage to the receiving system, followed by a DATA message.This tells the receiving systemthat the next part of the communication will be the actual body of the email message.Whenthe 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 re-ceiving system knows where to send the message.The protocol can verify that certain usersare indeed served by a particular mail server (the VRFY command) or expand a mailing list(theEXPNcommand).Email can also be relayed between two SMTP servers, if both systemspermit such activity.Unlike IMAP and POP, the SMTP protocol does not require authentication.This means thatSMTP servers can allow anyone on the Internet to use your system to send or relay mail tolarge lists of recipients.It is this characteristic of SMTP that makes spam possible.ModernSMTP applications attempt to minimize this behavior by restricting relaying and allowingonly known hosts to send email.RFC-821 outlines the basic behavior of SMTP, but several SMTP extensions, made possibleby RFC-1869, have added additional functionality to SMTP over the years by making newcommands available.By initiating a conversation with an SMTP server with an EHLO com-mand rather thanHELO, the connecting server can identify itself as one that supports SMTPextensions.The receiving server answers with a250line containing the various SMTP exten-sions it supports.Then, the connecting server can use the supported extensions as it wishesto accomplish the goals of the communication.One extension adds SMTP Authentication through the AUTH command as outlined in RFC-2554.Another widely used SMTP extension, detailed in RFC-2034, discusses the use of dot-separated, standardized error codes for use between SMTP applications.Reading the variousRFCs that involve SMTP provides a background to the way email moves around the Internet.In addition, you can connect to an SMTP server via telnet by specifying port 25, such astelnet localhost 25.Executing a few commands and sending a mail manually is a goodway to understand how SMTP communications work.Red Hat Linux 7.3 uses Sendmail as its SMTP program by default.However, easier to useapplications, such as Postfix, are also installed by default with the operating system.16.2.Email Program ClassificationsIn general, all email applications fall into at least one of three classifications.Each of theseclassifications plays a specific role in the process of moving and managing email messages.While most users are only aware of the specific email program they use to receive and sendmessages, each of these types is important for making sure email arrives at the correct desti-nation.16.2.1.Mail User AgentA Mail User Agent (MUA) is a program that, at the very least, allows a user to read andcompose email messages.An MUA is often referred to as an email client.Of course, manyMUAs help users do more than that, including retrieving messages via the POP or IMAPprotocols, setting up mailboxes to store messages, or helping present new messages to aMail Transfer Agent program that will deliver them to their final destination.MUA programs may be graphical, such as Mozilla Mail, or have a very simple, text-basedinterface, such asmuttorpine. 210 Chapter 16.Email16.2.2.Mail Transfer AgentA Mail Transfer Agent (MTA) transfers email messages between machines using SMTP.Amessage may involve several MTAs as it moves to its intended destination.Most users aretotally unaware of the presence of MTAs, even though every email message is sent throughat least one MTA.While the delivery of messages between machines may seem rather straightforward, theentire process of deciding if a particular MTA can or should accept a message for deliveryto a remote host is quite complicated.In addition, due to problems from spam, use of aparticular MTA is usually restricted by the MTA s own configuration or network access tothe system running it.Many of the larger and more complex MUAs can also be used to send email.However,this action should not be confused with the actions of a true MTA.In order for users notrunning their own MTA to move outbound messages off of their machine and onto a remotemachine for delivery, they must use a capacity in the MUA that transfers the message to anMTA they are authorized to use.However, the MUA does not directly deliver the messageto the intended recipient s email server that role is reserved for the MTA.Red Hat Linux uses Sendmail as its default MTA, though others may be used 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