title: Send webhook upon sending or receipt of email
body: <p>I have been looking around for how to create a webhook that is triggered when an email is sent or received in Gmail. For example someone sends an email, that email gets POSTed as a JSON object to one of my applications. I have found many middleman applications/services that do this (zapier offers all the functionality I want, but it costs money), but I can't find any documentation on how to actually do this myself so I don't need to pay for it.</p>
  
  <p>Is this possible?</p>