Lesson 16: Email
================

* Email service
    * How it works
    * Configuration Postfix
    * Planning

Email: System Components
------------------------

* Mail User Agent (MUA)
* Mail Transport Agent (MTA)
* Delivery Agent (MDA)
* Access Agent (MAA)

.. note:: 

    :MUA:
      - lets users read & compose mail
      - Thunderbird, mutt, etc
    :MTA:
      - routes messages to other machines
      - sendmail, postfix, exim, qmail
    :MDA:
      - places messages in local store
      - mail.local, procmail
    :MAA: access to mail store (i.e IMAP, POP)

Email: System Components
------------------------

.. figure:: static/email_routing.jpg
    :align: center

.. note::
  The most confusing part about email is understanding the routing.  Knowing the
  different components is important to fully grasping it.

Transport Agents
----------------

Accept mail form user agent

:Postfix:
  More common, easier to configure & use
:Sendmail:
  Highly configurable, steep learning curve

:Exim: Similar to Postfix
:Qmail: Logging is horrid, but some people like it

.. note:: 
    Postfix is the easiest to learn and understand, but queue management is a “black
    box”

    Sendmail & qmail is great for high volume sites, but postfix/exim still perform
    great.

    Sendmail has great options for queue management

    Features to look out for:
     * SASL (authenticated SMTP)
     * Queue Management

Delivery Agents & Message Stores 
--------------------------------

* procmail – great filtering
* maildrop – newer procmail-like
* mail.local
* Message Stores
    * mbox – one large file, locking problems
    * maildir – one file per message, great for IMAP

.. note:: 
    Consider scaling issues for the mailstore.

    Generally maildir is the best & most compatible option

Anatomy of a Mail Message
-------------------------

* Envelope
    * Destination email address
* Headers
    * Record of variety of important information
    * Great for tracking down problems
* Body of the message

.. note::
  Headers:
   * Know how to identify and track queue id's
   * Originator starts at the bottom
   * Headers can be forged
   * X- Headers non-RFC headers
   * Message ID is always unique

MTA Log Files
-------------

* Track emails via queue ID
    * Look something like: ``03CE18819A``
* Tracking via message ID
* Informational fields
    * to, from, status, relay, etc
* Log files differ between each MTA

.. note:: Being able to read log files is important.

Configuring Postfix
-------------------

* /etc/postfix
    * main.cf – main config file
    * master.cf – postfix process config file
    * /etc/aliases – local email forwarding
* Set to relay email to central MTA
    * relayhost = [smtp.osuosl.org]
    * myorigin = osuosl.org
    * /etc/aliases -- root: foo@gmail.com 

.. note:: 
    relayhost: [smtp.osuosl.org] vs. osuosl.org
     * [smtp.osuosl.org] goes directly to smtp.osuosl.org
     * 'osuosl.org' does DNS lookup and uses MX

    Make sure you run “newaliases” after updating /etc/aliases

    Reloading postfix is ideal too

    To test email: 
     echo “this is a test” | mail root@localhost

Sendmail
--------

* Config files created via m4
    * Makefile
* Always edit the .mc files not the .cf files
* Remember to rebuild .cf files with make
* Extremely configurable

.. note:: Config files in /etc/mail usually
    Primary file to edit should be sendmail.mc

Email: Viruses & Spam
---------------------

* Virus
    * Clamav 
    * Ensure freshclam is running too
* Spam
    * Spamassassin
* All-in-one
    * Amavis
* Check abuse emails

.. note:: 
    Make sure you have enough CPU & RAM for Spam checking
    Neglecting abuse emails may get you blacklisted
    For larger infrastructures, have dedicated machines to process spam
    Important to keep these updated

Email: Infrastructure Implementation
------------------------------------

* Small sites
    * Can have MTA/MDA/etc all on the same server
* Medium sites
    * Separate MTA from MDA
* Large sites
    * Split outgoing mail and incoming

.. note:: Consider resources, redundancy, & scalability.
    MDA is hardest to scale.

     * Look at Cyrus Murder for large scalability
     * dovecot is another option

Email: Security
---------------

* On General servers:
    * Only listen on localhost
    * Don't allow other hosts to relay through it
    * Relay all outbound mail through central host
* On Email servers:
    * Restrict relaying to trusted networks
    * Implement antivirus & spam protection

.. note:: Always test new configurations to ensure spammers can't relay mail
    through your server
    Having dedicate outbound servers will ensure they always catch spam/viruses/etc