Custom mail delivery method in Rails 3.*
----------------------------------------
Rails 3 allows you to specify a custom mail delivery method in addition to the default options smtp, sendmail, test and file. You can do this by specifying the class name of your mail delivery handler in the config file like this:

config.action_mailer.delivery_method = MyCustomDelivery

And the interface your class needs to implement is:

class CustomDelivery
  def deliver!(mail)
  end
end

How does this work?
-------------------
Say you have a mailer class for sending out a welcome email:

class Notifier < ActionMailer::Base
  def welcome
    mail(:to => "receiver@test.com",
         :from => "sender@test.com",
         :subject => "Testing custom mail delivery methods",
         :body => "Hello World")
  end
end

When you try to deliver a welcome email, the Mail object constructed in the "welcome" method is sent to the deliver! method of your delivery class.

mail = Notifier.welcome # Construct a mail object
mail.deliver            # Deliver the mail with the delivery method specified in the config

In your deliver! method you will then have full access to the mail object:

class CustomDelivery
  def deliver!(mail)
    puts mail.to       # receiver@test.com
    puts mail.from     # sender@test.com
    .
    .
    .
  end
end
  
Why is this useful?
-------------------
Consider a rails application like Twitter where different notification emails go out when people start following each other, send direct messages to each other, etc. Testing these emails by writing mailer tests is good. But it will also be nice to view the actual email that will be delivered to the users. To accomplish this we can write a custom SMTP (catch all) delivery class which inherits from the default Mail::SMTP class.

require 'rubygems'
require 'mail'

class CustomSmtpDelivery < ::Mail::SMTP
  # SMTP configuration (could be possible to pass the settings from the config file)
  def initialize(values)
    self.settings = {:address => "smtp.gmail.com",
                     :port => 587,
                     :domain => 'yourdomain',
                     :user_name => "username",
                     :password => "password",
                     :authentication => 'plain',
                     :enable_starttls_auto => true,
                     :openssl_verify_mode => nil
                    }.merge!(values)
  end
  attr_accessor :settings

  def deliver!(mail)
    # Redirect all mail to your inbox
    mail['to'] = "youremail@domain.com"
    mail['bcc'] = []
    mail['cc'] = []
    super(mail)
  end
end

In this class we first setup our SMTP configuration and in the deliver! method, we are basically redirecting all outgoing email to our personal email by modifying the "to field" and then passing on the modified mail object super class delivery method.

Usage
-----
1.) Update the SMTP configuration in lib/custom_smtp_delivery.rb to use your SMTP server
2.) Update the "to" email address in the CustomSmtpDeliver#deliver! method
3.) rails console
    > Notifier.welcome.deliver 
4.) Check your inbox.