title: Make an exception when validating emails
body: <p>I currently have an email validation as follows:</p>
  
  <pre><code>validates :email, presence: true, length: { maximum: 255 },
              format: { with: VALID_EMAIL_REGEX, message: "Must be your 
              &lt;domain name&gt; email address." },
              uniqueness: { case_sensitive: false }
  </code></pre>
  
  <p>This will work for all users of my application except one, an admin who does not have an email belonging to the required domain, and due to circumstances, I cannot create one for him.</p>
  
  <p>How can I go about allowing this admin to register an account with his email, bypassing the regular email validation?</p>