EM-Mailer is an asynchronous delivery implementation for ActionMailer that includes a mail client for your ActionMailer message deliveries and an eventmachine-based server-side process.

Delivering large numbers of e-mail messages synchronously is an exercise in disaster. And yes, there are lots of other implementations for asynchronous message delivery (ar_mailer, background job submission via a number of queue-based implementations, incl. MailHopper).

I personally wanted a lightweight daemon to do this for me, and didn't want to have to use my database or configure Resque or DelayedJob to handle these mass deliveries.

Enter eventmachine and em-mailer.

Getting em-mailer installed is straightforward. In your Gemfile:

gem 'em_mailer'

You must let ActionMailer know that you wish to use em-mailer as your delivery method. Either in your application's config/application.rb file (assuming Rails 3), or in an environment-specific config file such as config/environments/production.rb:

config.action_mailer.delivery_method = :em_mailer

Presuming you are sending e-mails via :sendmail that's all that is required for getting your Rails application configured.

If you want to use a different delivery method, or if you require additional delivery options, use the em_mailer_settings class attribute values.

For example, if you wish to use SMTP you can do something like this:

In config/initializers/em_mailer.rb

ActionMailer::Base.em_mailer_settings[:delivery_method] = :smtp
ActionMailer::Base.em_mailer_settings[:delivery_options] = {
  :address => '<< your smtp server name>>',
  :port => 25,
  :domain => '<< your domain >>',
  :authentication => :plain,
  :user_name => '<< user with access to smtp host >>',
  :password => '<< their password >>'
}

I have tested this with both a Rails 3.1 and Rails 2.x applications.

You'll need a running instance of the mail server running. The gem comes bundled with a binary called em_mailer_server. You can run it from the command line thusly:

em_mailer_server

To run it daemonized it's as easy as:

em_mailer_server -D

You can also specify the port that you want the server to listen on (default is 5600)

em_mailer_server -p 5600 -D

For a full list of configuration options pass it a -h, or --help.

I happen to be running em_mailer_server on a CentOS box. Here's my /etc/init.d/em_mailer script that I use to start|stop|restart the process on that host:

#!/bin/bash
#
# EM Mailer
#
# chkconfig: - 85 15
# description: start, stop, restart EM Mailer Server
#
RETVAL=0

case "$1" in
    start)
      /usr/local/bin/em_mailer_server -P /var/run/em_mailer.pid -D
      RETVAL=$?
  ;;
    stop)
      kill `cat /var/run/em_mailer.pid`
      RETVAL=$?
  ;;
    restart)
      kill `cat /var/run/em_mailer.pid`
      /usr/local/bin/em_mailer_server -P /var/run/em_mailer.pid -D
      RETVAL=$?
  ;;
    *)
      echo "Usage: em_mailer {start|stop|restart}"
      exit 1
  ;;
esac

exit $RETVAL

Lastly, I like to use monit to make sure my daemon processes are up and running. Here's my sample monit script:

check process em_mailer with pidfile /var/run/em_mailer.pid
  start program = "/etc/init.d/em_mailer start"
  stop program = "/etc/init.d/em_mailer stop"
  if failed port 5600 then restart
  group mail

Em::Mailer is Copyright (c) 2011 by Chris Kraybill and is provided under the MIT license. See COPYING for details.