Rails ActionMailer with multiple SMTP servers

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.