Sample code for how to attach an email when generating an email in Rails 3/4

contact_mailer.rb

class ContactMailer < ActionMailer::Base

  def receive(email)
    ContactMailer.contact_admins(email).deliver
  end

  def contact_admins(orig_email)
    m = mail(to: 'info@doorkeeper.jp', subject: 'Received email') do |format|
      format.text { render }
      format.html { render }
    end
    attach_original_email(m, orig_email)
  end

  def attach_original_email(m, orig_email)
    if m.parts.empty?
      m.convert_to_multipart
      m.send(:add_boundary)
      m.content_type = ["multipart", "mixed", m.content_type_parameters]
    else
      content_type_params = m.content_type_parameters
      part = Mail::Part.new
      part.content_type = ["multipart", "alternative", {}]
      2.times { part.add_part(m.parts.delete_at(0)) }
      m.content_type = ["multipart", "mixed", content_type_params]
      m.add_part(part)
    end
    m.attachments["original_email.eml"] = {:mime_type => 'message/rfc822', :content => orig_email.raw_source }
    m
  end
end