Electronic Mail

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Electronic Mail
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Electronic Mail

• Most successful Internet application
• Used to transmit messages from one person to
  another or from one person to many people
• Initially these were text only messages
• Today these messages make include pictures,
  advanced text markup (like different fonts and
  colors) or file attachments (like Word or Excel
  files)
• On the Internet, most email systems are built on
  a client, server, protocol architecture and
  follow Internet standards
• Some organizations use proprietary email
  systems that incorporate additional features like
  calendaring or workflow, well talk about them
  later

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Email Clients, Servers, Protocols

• The client is the email program the user utilizes
  (pine, elm, mail on Unix or Eudora, PC Pine,
  Netscape, Outlook, Outlook Express on PCs)
• In the case of email there isnt one email server
  and one email protocol, there are several servers
  and protocols. Each is responsible for a
  different part of the process.
• Generally speaking the mail sending and receiving
  function is handled by one service and protocol
  (SMTP)
• Another service and protocol is used to retrieve
  messages from the users mailbox so they can be
  displayed, deleted, etc. Those protocols are POP
  (Post Office Protocol) and IMAP (Internet Mail
  Access Protocol)

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Common Email delivery scenario
Mail server 2isp2.com
Mail server 1isp1.com
Stored Here untilUserConnects
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Delivery

• There are many variations on that scenario and we
  will discuss some of them later
• But first lets look at the pieces in more detail

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Start with the client

• This is the software the user uses to send and
  receive email. (Pine, Outlook Express, etc.)
• Gives you your user-interface and feature set
• Clients may be Unix based, PC or Mac based,
  dumb-terminal/character applications, GUI
  programs, or Web based
• No matter what they look like on the outside, on
  the inside they all have to do the same thing to
  send mail on the Internet which is..
• They all must talk SMTP

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Email Client Features

• Common
• alias and alias-lists, (address books)
• folders
• forwarding, carbon copies (cc/bcc), reply
• file inclusion, attachments
• signature file...signature-file
• Not there yet
• Security (PGP, S/MIME, possible technologies but
  not widely used)
• Authentication (Digital certificates possible
  technologies but again not widely used)

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Email Addresses

• Internet standard
• userid_at_host.domain
• such as barker_at_u.washington.edu
• Some variations are possible like
• Scott.Barker_at_xyz.com
• 12345.65362_at_xyz.com
• In the old days many people in education used
  something called Bitnet
• Addresses looked like barker_at_washington or
• Barker_at_washington.bitnet
• Used a store and forward mechanism to deliver
  mail
• In the commercial world there were competing
  alternatives like MCIMail, CompuServe each had
  its own address format and couldnt talk to the
  other!

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Finding Email Addresses

• Usually you just ask!
• Were some crude old tools like
• finger (UNIX)
• Netfind
• telnet ds.internic.net not there anymore
• Recently have appeared servers on the web that
  let you look for email addresses.
• www.bigfoot.com, people.yahoo.com
• Often they support a protocol called LDAP
  (Lightweight Directory Access Protocol) that
  allows you to query them inside your mail client.
  
• UW offers an LDAP service that can be very
  useful.

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Email Clients - Unix Pine

• At the Unix prompt type pine
• Note Where is this program running (on your PC
  or on the Unix machine)?
• Does the mouse work? (Why or why not)?
• C for compose a message
• Fill in To, Subject
• Return takes you into message body
• Ctrl-X to send it (see bottom of screen)
• Send a message to your neighbor
• Note the TO line syntax of user_at_host.domain

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Additional Basic Pine functionality

• Inbox
• Deleting Messages
• Using Folders
• Reply forward
• Address book
• Play and experiment on your own if interested

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How about the original?

• Pine is actually a very sophisticated email
  program for Unix and Pine was developed here at
  UW. Before Pine many used the standard Unix mail
  program called mail, on Dante there is a
  similar program still available called mailx
• In this weeks exercise I ask you to try the
  mailx program
• You will see that there are very limited editing
  capabilities, and some no so intuitive things
  for example you end a message by placing a . at
  the beginning of a new line
• Obviously mail didnt have the best UI.
  Another popular Unix mail program called elm
  became popular at many sites. Pine stands for
  ...

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Delivering the message

• Once you have composed and sent an email
  message on a Unix based system, your message is
  usually placed in an outgoing mail queue folder
• A server process running on your box checks that
  folder periodically and if it finds an outgoing
  message, it initiates an SMTP connection to the
  destination mail server
• The most commonly used SMTP server software on
  Unix is probably sendmail although there are
  many other options
• If the destination SMTP server is up and running
  accepting incoming connections, the message will
  be delivered immediately
• If the destination is not accepting SMTP incoming
  connections the message will be placed back into
  the outgoing mail folder and queued-up for
  another attempt (usually 10-15 minutes later)
• If the message cannot be delivered after some
  period of time (typically 3 days) it is returned
  to the sender as undeliverable

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SMTP

• Simple Mail Transport Protocol
• Originally defined by RFC 821 in 1982 with a
  modified standard (RFC 2821) proposed in 2001
• Defines a host to host mail transport and
  delivery protocol
• SMTP servers may be the source or final
  destination for a message or they may relay
  mail for others.
• Today mail relaying is often allowed in only
  limited situations, i.e. an SMTP server like UWs
  server may only relay mail for machines that
  connect to it from the UW Network. This prevents
  spammers from using our server to distribute
  their mail.

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SMTP Details

• SMTP servers connect and listen on port 25
• SMTP is a text based protocol with commands
  like
• HELO (or EHLO in the newest version)MAIL
  FROMRCPT TO
  DATAQUIT

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Seeing the Protocol in Action

• The mailx program has a -v (for verbose) option
  that allows you to watch the SMTP protocol in
  action.
• If I used that option I would see something like
  (see next page)

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homer04 mailx -v scott_at_ils.unc.edu Subject
Test This is a test message . Cc scott_at_ils.unc.ed
u... Connecting to mailscan-out1.cac.washington.ed
u. via relay.. . 220 mailscan-out1.cac.washington.
edu WebShield SMTP V4.5 MR1a Network Associates ,
Inc. Ready at Sun Sep 08 132706 2002 EHLO
homer04.u.washington.edu 502 Command not
Supported HELO homer04.u.washington.edu 250
mailscan-out1.cac.washington.edu Welcome
homer04.u.washington.edu MAIL
From 250
... OK RCPT
To 250
... OK DATA 354 Enter mail, end with "." on a
line by itself . 250 Mail accepted scott_at_ils.u
nc.edu... Sent (Mail accepted) Closing connection
to mailscan-out1.cac.washington.edu. QUIT 221
mailscan-out1.cac.washington.edu closing
Connection homer04
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Faking mail.

• Since SMTP is a text based protocol, it is
  possible to connect directly to the SMTP port on
  a mail server and talk SMTP.
• The well known port for SMTP is 25
• telnet mailhost.u.washington.edu 25
• In exercise 7 I have asked you to do this. You
  will be acting as a mail client and issuing SMTP
  commands directly to the server

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Looking at our fake message

• If we looked at our message in Pine or Outlook,
  could we tell it is a fake message?
• We can turn on full headers in Pine to see
  more.
• Setup, Config, check the option to
  enable-full-header-command
• Then when viewing a message hit H to see the full
  header
• Note Not all SMTP servers are configured to do a
  reverse lookup of the sending machines IP
  address (recall the pleased to meet you line.
  And that the UW server noticed the real machine
  we were connecting from.) Without that, a
  forgery could be completely hidden even if you
  looked at the full headers of the mail

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More on SMTP

• The S in SMTP stands for SIMPLE
• SMTP needs add-ons for
• security/privacy (PGP or S/MIME)
• binary attachments (MIME)
• Again, SMTP is only involved in the delivery of
  mail from one mail server to another. It has
  nothing to do with how you read the mail once it
  arrives.
• POP and IMAP are protocols for PC/Mac based
  clients to retrieve mail off the server once it
  has arrived.
• Well discuss those later

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Where does mail go whenit gets to the
destination mail server?

• Remember that SMTP simply delivers mail. As it
  arrives a couple things have to happen.
• First there is a check - does this mail need to
  be forwarded elsewhere? On Unix there is an
  /etc/aliases file for the administrator to setup
  forwarding
• Alternatively on some systems the user can do it
  themselves by creating their own .forward file.
  In that the user enters the address they want to
  forward to. Other systems use different
  forwarding options, for example on Dante you
  select forward from their main menu when you
  login
• One caution, if you forward, be careful not to
  create forwarding loops!

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If it isnt forwarded

• Mail has to be placed somewhere so the user or
  the mail client can retrieve it later
• On Unix there is typically a special directory
  where incoming mail is stored. Often this is
  /var/mail
• Typically each user has a single file in
  /var/mail and all their messages are appended
  together into that one file with each message
  separated by a header (From and To
  information)
• On Dante the location where your mail is
  delivered is actually hidden from you so they can
  move things around as needed to distribute the
  load
• On PC based servers mail also usually just goes
  into a folder, or alternatively it may be added
  to a database for later retrieval.
• The precise delivery location on either platform
  depends on the server software that is running
  and how that mail server has been configured by
  the system administrator
• End users normally would not look in these
  locations themselves, instead a mail client will
  either access the location through the file
  system, or use a protocol like POP, or IMAP to
  request messages from the server and display them
  to the user

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The entire mail delivery process

• User uses a mail client to compose
• That client turns the message over to a mail
  server that will try and deliver the mail
• The mail server uses the SMTP protocol to talk to
  another mail server at the destination
• The destination mail server also talks SMTP and
  it either accepts or forwards the mail elsewhere
• If mail stays at destination, the destination
  server stores the message waiting for the user to
  read it
• Destination user runs their mail client which
  checks their inbox for new mail, parses
  messages, and provides nice or not nice user
  interface! This is done using another protocol,
  POP or IMAP.

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Back to the diagram
sue_at_isp2.com
mary_at_isp1.com
SMTP
Why do we need intermediary SMTP servers to
deliver mail and additional protocols
for retrieving and displaying messages from a
mail server? Why not just connect directly
to the destination machine to deliver the mail
our host/PC to their host/PC ?
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Mail exchange without an intermediary.

• Possible but.
• What address do we use? Recall that many hosts
  have dynamic IP addresses and most PCs do not
  have IP names that are registered in DNS
• What if the destination is not on or up?
• What if the destination is not running an SMTP
  server that is listening for incomming
  connections?
• How do we effectively queue messages up for later
  delivery since our own machine may be powered off
  on occasion?
• What if we turn off our machine, how will we
  receive new mail?
• As a result of these and other issues, PCs email
  clients are configured with an SMTP Server that
  will act as an intermediary for sending messages.
  They also are configured to use a POP or
  IMAP server to receive incoming messages.
  Sometimes these may be the same box, sometimes
  they are different.

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End of Part 1 on email