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					Mail Services
A Mail Server’s Function
   A mail server has basically 4 Functions

  1. Accept Mail (From other servers)
  2. Send Mail (To a remote server)
  3. Accept Mail (From one user to be sent
     out to another server)
  4. Send mail (To end user)
 A sample Message

         Outgoing      SMTP   Incoming      POP or
 SMTP    mail server          mail server   IMAP


User A                                      User B
The Messaging process
 When a message is sent to a mail server it
 uses SMTP (Simple Mail Transfer Protocol)
 to send the message.
 SMTP uses port 25 on the accepting mail
 server.
 When the message arrives at the
 destination server it is then downloaded by
 a mail client using either POP3 or IMAP
 protocols
POP
 Post Office Protocol
 When you read your mail, all of it is
 immediately downloaded to your
 computer and no longer maintained on
 the server.
IMAP
 Internet Message Access Protocol
 protocol in which e-mail is received and
 held for you by your Internet server.
 Your e-mail client can view just the
 heading and the sender of the letter
 and then decide whether to download
 the mail
Features of IMAP
  You can also create and manipulate
  folders or mailboxes on the server,
  delete messages, or search for certain
  parts or an entire note.
  IMAP requires continual access to the
  server during the time that you are
  working with your mail.
IMAP Vs POP
 IMAP can be thought of as a remote file
 server.
 POP can be thought of as a "store-and-
 forward" service.
Common Email Port Addresses
  110 pop3

  25 smtp

  143 imap
Relaying Messages
  Relaying is the process of sending or
  forwarding the message.
  Most mail servers will restrict where the
  forward messages from this is to reduce
  spamming.
  This is accomplished by setting a
  number of rules stating who is
  authorized and who isn’t.
What makes a user authorized?

              Outgoing      •IP address
              Mail Server   •Login/password

Authorized    BLOCKED       •The sender’s domain
user                        •The sender’s email
                            address
             Unauthorized   •The reply to address
             user           matches the sender’s
                            address
                            •The receiver's
                            address/domain
Different Unix Mail Servers
  Sendmail
  Qmail
  POSTFIX
  SMAIL
  EXIM
  MMDF
  ZMAILER
Other Mail Servers
  Exchange - Microsoft
  GroupWise - Novell
  Lotus notes – IBM
The SMTP Protocol
  HELO hotmail.com bobo@bobo.com
   Greet the mail server. Used once per
   session - at the beginning of the session.
The SMTP Protocol
  MAIL FROM: <from>
   Announce who the sender is. Used once
   per mail, before specifying any recipients
   for each mail, or after a RSET.
The SMTP Protocol
  RCPT TO: <rcpt>
   Announce who the mail is to. Multiple
   recipients are allowed, each must have its
   own RCPT TO: entered immediately after a
   MAIL FROM:
The SMTP Protocol
  DATA
   Starts mail entry mode. Everything entered
   on the lines following DATA is treated as
   the body of the message and is sent to the
   recipients. The DATA terminates with a .
   (period) on a line by itself. A mail may be
   queued or sent immediately when the . is
   entered. It cannot however be reset at this
   stage.
The SMTP Protocol
  RSET
   Reset the state of the current transaction.
   The MAIL FROM: and RCPT TO: for the
   current transaction are cleared.
  QUIT
The SMTP Protocol
  DATA

  From: me@mymachine
  To: bobo@bobo.com
  Subject: A test message

  This is gonna be like a small info bulletin on
  how... ... .
A sample SMTP Session
homer:~$ telnet mx06.hotmail.com 25
Trying 64.4.55.7...
Connected to mx06.hotmail.com.
Escape character is '^]'.
220-HotMail (NO UCE) ESMTP server ready at
  Thu, 14 Feb 2002 05:52:19 -0800
220 ESMTP spoken here
A sample SMTP Session
  helo fred@fred.com
250 Requested mail action okay, completed
  mail from: bigelow@rcc.on.ca
250 Requested mail action okay, completed
  rcpt to: rbigelow@hotmail.com
250 Requested mail action okay, completed
  data
354 Start mail input; end with <CRLF>.<CRLF>
A sample SMTP Session con’t
  From: bigelow@rcc.on.ca
  To: bobo@hotmail.com
  Subject: Hello

  This is a test
  .
250 Requested mail action okay, completed
  quit
The POP Protocol
  telnet popserver.ca 110
  +OK QPOP (version 2.53) at
  popserver.com starting.
  <1106.1075384691@popserver>
  USER username
  +OK Password required for hacker.
  Pass password
POP Protocol
  LIST _shows messages
  RETR – Downloads Messages
  DELE – Deletes a Message

				
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