Use Exim and pipes to replicate MailGun routes

Over the last month or so I’ve been working with a friend of mine on a little side project (more on that later). During the course of the project, we ended up using Mailgun to receive email which was then passed to a PHP script using Mailgun routes.

Mailgun was pretty easy to setup and things were great but unfortunately in the last day or so our free app started creeping over the 200 message/day limit on the Mailgun free tier. Since the app is currently free, we couldn’t really justify paying the $19/mon minimum for the first Mailgun tier so I started exploring self-hosted options.

After a big of Googling, the first thing that caught my eye was the LamsonProject. From my understanding, Lamson basically provides a SMTP server which binds to a MVC application framework. After checking out the docs and the code it seemed like Lamson would allow you to build a RESTful application that responded to SMTP messages instead of HTTP requests. It certainly seems like a cool project but seemed like a bit overkill for what I needed to do.

Going further down the rabbit hole, I found out that the Exim allows messages to be delivered to a regular *nix process via a pipe and that the configuration was supposedly relatively straightforward under Ubuntu. Given that, I decided to give it a shot, it turned out to be relatively easy to setup but here is a quick rundown.

Step 1: Setup Exim

By default, Ubuntu systems are configured with sendmail and only setup to receive local mail. You’ll need to remove sendmail, install Exim, and then configure it.

To do this you’ll roughly need to run the following:

sudo apt-get remove sendmail
# for some reason the dependencies aren't being installed correctly
# see http://ubuntuforums.org/showthread.php?p=10411454
sudo apt-get install exim4-base exim4-config
sudo apt-get install exim4
sudo dpkg-reconfigure exim4-config

Next, follow the guide on https://help.ubuntu.com/8.04/installation-guide/hppa/mail-setup.html to configure Exim.

Step 2: Configure Pipes and Aliases

Per https://answers.launchpad.net/ubuntu/+source/mailman/+question/120447 it turns out something isn’t enabled in the default Ubuntu configuration of Exim. To solve this, I added the following

.ifndef SYSTEM_ALIASES_PIPE_TRANSPORT
SYSTEM_ALIASES_PIPE_TRANSPORT = address_pipe
SYSTEM_ALIASES_USER = Debian-exim
SYSTEM_ALIASES_GROUP = daemon
.endif

Into the bottom of /etc/exim4/conf.d/transport/30_exim4-config_address_pipe

After you edit an Exim setting, you’ll need to always run the following for the settings to take effect:

sudo update-exim4.conf.template -r
sudo update-exim4.conf
udo service exim4 restart

Next, you’ll need to enable the catch-all router configuration. This was surprisingly difficult to track down how to do:

Per, http://pontus.ullgren.com/view/Setting_SMTP_server_development_test_server_running_Ubuntu you need to create a file at /etc/exim4/conf.d/router/950_exim4-config_catchall containing

catch_all:
   debug_print = "R: catch_all for $local_part@$domain"
   driver = redirect
   data = ${lookup{*}lsearch{/etc/aliases}}
   # NOTE: I added this line for the catch all pipes to work
   pipe_transport = address_pipe

Finally, add a catch-all alias that pipes to a script to your /etc/aliases file. Mine looks like:

*: "|/home/ubuntu/exim4.php"

Step 2: The PHP script

I did this with PHP since my original Mailgun script was in PHP but anything should work. Key notes, the script needs to be accessible AND executable by the Exim user. Also, you’ll need the appropriate shebang to make the script work without a named interpreter.

Your script is going to receive raw SMTP email so you’ll need to parse that out to do anything meaningful with it. Thankfully there are plenty of libraries to do this. I ended up using MailParse along with PHP MimeMail Parse to abstract the nitty gritty into some cleaner object oriented code.

The part of my script that accepts and parses the email looks like:

From there, you’d be free to do anything you wanted with $msgInfo, just like on the receiving end of a Mailgun route.

Anyway, as always let me know if you have any feedback, questions, or comments.

Posted In: Demo, Tips n' Tricks

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