title: How can I set the value of 'config.action_mailer.default_url_options' with domain.name?
body: <p>I have to setup this for my devise usage.<br>
  But my app allows users to access through 2 domains.<br>
  I wanna set 'config.action_mailer.default_url_options = { :host =>' with the domain that users are accessing to.  </p>
  
  <p>How can I do this?</p>