actionmailer_extensions.rb

module ActionMailer
  class Base
    def perform_delivery_from_header(tmail)
      method = tmail.header.delete("x-delivery-method") || :server_2
      send "perform_delivery_#{method}", tmail
    end

    def perform_delivery_server_1(tmail)
      self.class.smtp_settings = {
          address: "server1.smtp.com",
          port: 25
      }
      perform_delivery_smtp tmail
    end

    def perform_delivery_server_2(tmail)
      self.class.smtp_settings = {
          address: "server2.smtp.com",
          port: 587
      }
      perform_delivery_smtp tmail
    end
  end
end

actionmailer_extensions_spec.rb

require 'spec_helper'

class TestMailer < ActionMailer::Base
  self.template_root = Rails.root.join("spec/mailer_views").to_s

  def test_mandrill_smtp
    from "me@example.com"
    recipients "you@example.com"
  end

  def test_sendgrid_smtp
    from "me@example.com"
    recipients "you@example.com"
    headers "X-Delivery-Method" => "sendgrid_smtp"
  end
end

describe ActionMailer::Base do
  before do
    ActionMailer::Base.delivery_method = :from_header
    Net::SMTP.stubs(:new)
  end

  after { ActionMailer::Base.delivery_method = :test }

  it "delivers via mandrill" do
    TestMailer.any_instance.expects(:perform_delivery_smtp)
    TestMailer.deliver_test_mandrill_smtp
    TestMailer.smtp_settings[:address].should == "smtp.mandrillapp.com"
  end

  it "delivers via sendgrid" do
    TestMailer.any_instance.expects(:perform_delivery_smtp)
    TestMailer.deliver_test_sendgrid_smtp
    TestMailer.smtp_settings[:address].should == "smtp.sendgrid.infos"
  end
end

production.rb

config.action_mailer.delivery_method = :from_header