Email System

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Email System
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Internet Relay Email
From P. 447, of TCP/IP Illustrated Volume 1,
Richard Stevens
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Mail message format

• SMTP protocol for exchanging email msgs
• RFC 2822 standard for text message format
• envelope
• header lines, e.g.,
• To
• From
• Subject
• body
• the message, ASCII characters only

header
blank line
body
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Functions of Mail Servers

• To accept outgoing mail from user agents and
  inject it into the mail system
• To receive incoming mail from the outside world
• To deliver mail to end users mailboxes
• To allow users to access their mailboxes with
  IMAP or POP

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Mail System Architecture
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A Message from Eric
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A Message from Eric
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Final Delivery
(a) Sending and reading mail when the receiver
has a permanent Internet connection and the user
agent runs on the same machine as the message
transfer agent. (b) Reading e-mail when the
receiver has a dial-up connection to an ISP.
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Aliasing by Example
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Software Packages for Maintaining Mailing Lists

• Majordomo
• http//www.greatcircle.com
• Mailman
• http//www.list.org/
• ListProc
• http//www.cren.net/
• SmartList
• Derived from procmail http//www.procmail.org/
• Listmanager
• http//www.listmanager.org/
• LISTSERV Lite
• www.lsoft.com

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Sendmail

• www.sendmail.org
• Tasks
• Controlling messages as they leave the users
  keyboard
• Understanding the recipients addresses
• Choosing an appropriate delivery or transport
  agent
• Reformatting headers as required
• Passing the transformed message to the delivery
  agent
• Players
• sendmail.cf
• Control incoming mail and mail queues during the
  submission process
• submit.cf
• Control the handling of mail that originates on
  the local machines
• Mail queues /var/spool/mqueue,
  /var/spool/clientmqueue
• Other files

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Sendmail Configuration File

• To determine
• Choice of delivery agents
• Address rewriting rules
• Mail header formats
• Options
• Security precautions
• Spam resistance

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Spam-related Features in Sendmail

• Relaying
• The access database
• User or site blacklisting
• FEATURE(blacklist_recipients)
• www.spamhaus.org
• Header checking
• FEATURE(accept_unresolvable_domains) sendmail
  accepts mail from domains that do not exist or do
  not resolve
• Rate and connection limits set per-host or
  per-net limits on the rate at which incoming
  connections are accepted
• SpamAssasin
• http//spamassassin.apache.org/

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One Example Access Database
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One Example Access Database