title: Allow customers to send from their own domain in a SAAS application
body: <p>I'm currently running a SAAS application and mails are being sent from our application using Mailjet.</p>
  
  <p>Some of the larger customers have been asking to allow the emails to be sent from their domain (e.g. info@largehotel.com) instead of our system (notifications@saasapp.com).</p>
  
  <p>Are there any initial pointers I will need to look at? I'm guessing they will need to add our SPF records to their SPF records too and that they will need add a DKIM key that we generate for them to add to their records too? Then do some validation on them on the DNS level and mark them as validated?</p>
  
  <p>I have some understanding to have customers run their own domain against our SAAS domain but a bit lost on the sending from their email domain requirement.</p>