title: Check if email domain is valid
body: <p>I have an input field for email_id in my form.<br>
  When the user enters the email id and submits the form, i need to validate the domain of the email.</p>
  
  <p>(Note: I need not validate the email address. I need to Validate only the Domain)</p>
  
  <p>What is the best way to check whether the Email-Domain is Valid or Not in Ruby on Rails?</p>