Main Site --> www.mysite.com
   Iframe --> www.myiframe.com

<system.net>
    <mailSettings>
      <smtp from="me@mysite.com" deliveryMethod="Network">
        <network host="smtp.office365.com" userName="me@mysite.com" password="password" defaultCredentials="true" port="587" enableSsl="true" />
      </smtp>
    </mailSettings>
  </system.net>