Thoughts on Postfix/Imap/Squirrelmail ­ Building a complete Mail Server.

Introduction:
This discussion will touch on the main points and operation of the Postfix Mail Transport Agent (MTA) and how it can interact with the Courier-Imap server and Squirrelmail web access scripts.

What is Postfix?
Postfix is a Mail Transfer Agent (MTA) (Email Server) for Unix/Linux and various other operating systems. It was developed as a more secure and easier to use alternative to sendmail. It started out as a secure mail server for IBM. The main programmer and maintainer of Postfix is Weiste Venema

Is Postfix everything I need to have a complete mail server?
Postfix does not have integrated services such as POP or IMAP to read mail. Several POP/IMAP servers exist that can cooperate with Postfix. Postfix by itself can deliver mail to other Mail Servers or receive mail from other Mail servers. When Postfix receives mail from other mail servers it stores this mail in specified directories but does not deliver it or provide access to the mail to regular users.

A Mail User Agent (MUA) is needed to access these mailboxes. A mail user agent can be as simple as the pine email program checking email delivered to a local mailbox. If you want to check your mail on a remote mail server you will need to use a mail user agent protocol such as IMAP (Interactive Mail Access Protocol) or POP. Most email client programs (otherwise known as Mail User Agents) communicate over the IMAP or POP protocols to receive email from where the Postfix MTA placed it. There are also webmail interfaces today that allow a user to access her/his mail directory from a website.

Understanding just how Postfix Processes Mail:
Postfix breaks down the process of delivering mail into many smaller parts. In a nutshell, either a message starts out internally to the network or starts externally coming into the network. If the mail starts out internally the travel path would look something like this:

local machine->sendmail->postdrop->maildrop->pickup->cleanup

If the message starts out externally it proceeds right to the cleanup process. The cleanup process makes sure all of the email header information is in order. Then it passes the message to the nqmgr, which is a process that decides just where the mail should go. Three of the more typical destinations include 1. a local mailbox, 2. an eternal smtp server, or 3. a bounce, or deferred directory. Bounced messages are sent back to the nqmgr for processing back to the original sender. Deferred messages are kept for a certain time period and sent back through nqmgr in hopes of a successful future delivery.

The various programs above that process the mail send the mail through different queues. These directories can be found in the /var/spool/postfix. By default Postfix will place the delivered mail in the /var/spool/mail directory. This default mail directory can be changed in the main.cf file.

Configuration Files.

Looking at the configuration files for Postfix: Two main config files, master.cf and main.cf

The master.cf file controls basic behaviors of the core processes. Be very careful how you manipulate the data here. By default the Redhat and Mandrake rpms allow most core processes to operate in a chroot jail. To get Postfix running you should not have to edit anything in this file, especially when installed via rpm.

There are hundreds of parameters that can go in the main.cf directory. Be sure to back up the default file as it is nicely documented.

Where everything is stored:
queue_directory = /var/spool/
postfix command_directory = /usr/sbin
daemon_directory = /usr/lib/
postfix mail_owner = postfix

What the hostname and domain name is, where applicable:

myhostname = bob.sample.org (This is used as the default address for all local users).
mydomain = sample.org

If the hostname is the same as the domain name, then the mydomain is the only variable needed.

myorigin = $mydomain ­ Determines the format of the origin address for all messages sent by postfix

inet_interfaces = all ­ What interfaces Postfix will accept connections on.

mydestination = $myhostname, localhost.$mydomain The final name to accept mail for.

***Important***

home_mailbox = Maildir/ - This tells Postfix to deliver to the /home/user/Maildir directory. This is necessary so that courier-imap can access the delivered mail.

Understanding Log Files (/var/log/mail/info)

May 19 00:21:50 sample postfix/smtpd[1352]: 90C0126ECAE: client=someone.mchsi.com[204.xxx.xxx.xxx]

May 19 00:21:51 sample postfix/cleanup[1353]: 90C0126ECAE: message-id=3EC875C3.2010500@mchsi.com

May 19 00:21:51 sample postfix/nqmgr[3296]: 90C0126ECAE: from= <someone@mchsi.com>, size=2666, nrcpt=3 (queue active)

May 19 00:21:51 sample postfix/local[1357]: 90C0126ECAE: to=<bob@sample.org>, relay=local, delay=1, status=sent (maildir)

May 19 00:21:52 sample postfix/smtpd[1352]: disconnect from someone.mchsi.com[204.xxx.xxx.xxx]

Other neat easy things to do with Postfix:

Aliases can be set up in /etc/postfix/aliases In the form mike: michael, bob (The alias mike is delivered to both user michael and bob)

Virtual Domains can be set up in /etc/postfix/virtual

In the form mike@mikesite.com: mike

(this is just one way to do it for a small number of users. Proper dns service will also have to be configured as well.)

UCE (Spam Controls)

I've not used these yet. Out of the box Postfix is not an open relay.

Where do I get the Postfix software?

The software is compiled into almost every Linux distribution, finding it will not be hard at all. The source can be found directly from postfix.org

What are the latest versions?

RH9: postfix-1.1.11-11.i386.rpm
RH8: postfix-1.1.11-5.i386.rpm

Mandrake 9.1: 2.0.6-1mdk.i586
Mandrake 9.0: postfix-1.1.11-4mdk.i586.rpm

Adding Courier-Imap as the MUA (Mail User Agent Protocol)

If you can find a rpm or packaged install, this will be easiest. :-) Courier is actually an entire mail server program, you'll just need the courier-imap portion which can be downloaded separately. Courier-imap includes a pop3 server and an imap server.

The download instructions as well as instructions for making the rpm or installing from a tarball can be found at courier-mta.org. Installing from a tarball isn't that hard, it actually worked flawlessly on my RH9 Server. Being able to use an rpm though will create all of the boot scripts that will make courier-imap work right off of the install. :-)

Courier-imap requires a Maildir format to retrieve mail from. A mailbox will need to be created for each user, in his or her home directory. The command to use is:

/usr/lib/courier-imap/bin/maildirmake Maildir

This command can be run in each home directory (as each user, not as root). A Maildir can be added to /etc/skel so that it is automatically created for each new user. Remember that Postfix needs to be configured to deliver to the Maildir directory instead of the default delivery location.

Adding Squirrelmail as a Web-based email client.

Squirrelmail is very easy to set up. Out of the box it has most everything needed to be fully functional. Squirrelmail is a set of php files and scripts along with a web page framework that can be dropped right in to an existing website. The configuration script is very straightforward. It contains default values for several different imap servers and Linux distributions. Be sure to enable php support in your web server, and also allow: HTTP file uploads. file_uploads = On (From the /etc/php.ini file)

Other Comments

Practice with security in mind. Either block ports properly at your firewall or keep the test mail server off of the network.

You can also configure most services only to respond on certain interfaces/ports.

Join the discussion lists, or peruse the extensive online archives/faqs for answers to common problems.

If you are thinking about setting up Squirrelmail you may also want to make that part of your website secure so the passwords aren't passing back and forth in clear text.

Resources

Books

Postfix ­ Richard Blum

The Book of Postfix ­ Ralf Hildebrandt, Patrick Koetter (Not yet released)

An O'Reiley Postfix Book ­ Kyle Dent (Not yet released)

Links

http://www.postfix.org/faq.html
http://www.postfix.org/uce.html

http://www.courier-mta.org/
http://squirrelmail.org/

Great Compilation Doc:
http://www.firstpr.com.au/web-mail/RH71-Postfix-Courier-Maildrop-IMAP/

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