Rails ActionMailer with multiple SMTP servers

Question

Based on the Oreilly article, I came up with the solution I wrote about here: http://transfs.com/devblog/2009/12/03/custom-smtp-settings-for-a-specific-actionmailer-subclass

Here's the relevant code:

class MailerWithCustomSmtp < ActionMailer::Base
  SMTP_SETTINGS = {
    :address => "smtp.gmail.com",
    :port => 587,
    :authentication => :plain,
    :user_name => "[email protected]",
    :password => 'password',
  }

  def awesome_email(bidder, options={})
     with_custom_smtp_settings do
        subject       'Awesome Email D00D!'
        recipients    '[email protected]'
        from          '[email protected]'
        body          'Hope this works...'
     end
  end

  # Override the deliver! method so that we can reset our custom smtp server settings
  def deliver!(mail = @mail)
    out = super
    reset_smtp_settings if @_temp_smtp_settings
    out
  end

  private

  def with_custom_smtp_settings(&block)
    @_temp_smtp_settings = @@smtp_settings
    @@smtp_settings = SMTP_SETTINGS
    yield
  end

  def reset_smtp_settings
    @@smtp_settings = @_temp_smtp_settings
    @_temp_smtp_settings = nil
  end
end

class UserMailer < ActionMailer::Base
  def welcome_email(user, company)
    @user = user
    @url  = user_url(@user)
    delivery_options = { user_name: company.smtp_user,
                         password: company.smtp_password,
                         address: company.smtp_host }
    mail(to: @user.email,
         subject: "Please see the Terms and Conditions attached",
         delivery_method_options: delivery_options)
  end
end

Rails 4 allows for dynamic delivery options. The above code is straight from the action mailer basics guide, which you can find here: http://guides.rubyonrails.org/v4.0/action_mailer_basics.html#sending-emails-with-dynamic-delivery-options

With this, it is possible to have different smtp settings for every email you send, or, like in your use case for different sub classes like UserMailer, OtherMailer etc.

Solution for Rails 4.2+:

config/secrets.yml:

production:
  gmail_smtp:
    :authentication: "plain"
    :address: "smtp.gmail.com"
    :port: 587
    :domain: "zzz.com"
    :user_name: "[email protected]"
    :password: "zzz"
    :enable_starttls_auto: true
  mandrill_smtp:
    :authentication: "plain"
    :address: "smtp.mandrillapp.com"
    :port: 587
    :domain: "zzz.com"
    :user_name: "[email protected]"
    :password: "zzz"
    :enable_starttls_auto: true
  mailgun_smtp:
    :authentication: "plain"
    :address: "smtp.mailgun.org"
    :port: 587
    :domain: "zzz.com"
    :user_name: "[email protected]"
    :password: "zzz"
    :enable_starttls_auto: true

config/environments/production.rb:

config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = Rails.application.secrets.gmail_smtp

app/mailers/application_mailer.rb:

class ApplicationMailer < ActionMailer::Base
  default from: '"ZZZ" <[email protected]>'

  private

  def gmail_delivery
    mail.delivery_method.settings = Rails.application.secrets.gmail_smtp
  end

  def mandrill_delivery
    mail.delivery_method.settings = Rails.application.secrets.mandrill_smtp
  end

  def mailgun_delivery
    mail.delivery_method.settings = Rails.application.secrets.mailgun_smtp
  end
end

app/mailers/user_mailer.rb:

class UserMailer < ApplicationMailer
  # after_action :gmail_delivery, only: [:notify_user]
  after_action :mandrill_delivery, only: [:newsletter]
  after_action :mailgun_delivery, only: [:newsletter2]

  def newsletter(user_id); '...' end # this will be sent through mandrill smtp
  def newsletter2(user_id); '...' end # this will be sent through mailgun smtp
  def notify_user(user_id); '...' end # this will be sent through gmail smtp
end

For anybody approaching this issue with later versions (3+) of Rails, try this

http://guides.rubyonrails.org/action_mailer_basics.html#sending-emails-with-dynamic-delivery-options

Tried to use jkrall's option with Rails 3.2.1 but for some reason it wouldn't override default configuration, but doing:

MyMailer.my_email.delivery_method.settings.merge!(SMTP_SETTINGS).deliver

Similar to http://www.scottw.com/multiple-smtp-servers-with-action-mailer, made it work.

Rails-2.3.*

# app/models/user_mailer.rb
class UserMailer < ActionMailer::Base
  def self.smtp_settings
    USER_MAILER_SMTP_SETTINGS
  end

  def spam(user)
    recipients user.mail
    from '[email protected]'
    subject 'Enlarge whatever!'
    body :user => user
    content_type 'text/html'
  end
end

# config/environment.rb
ActionMailer::Base.smtp_settings = standard_smtp_settings
USER_MAILER_SMTP_SETTINGS = user_smtp_settings

# From console or whatever...
UserMailer.deliver_spam(user)