title: Can App Engine MailService be used with OAuth2 / service accounts?
body: <p>We are using UserService with OpenId to send emails on behalf of the logged in user.
  As we want to move to OAuth2/OpenID Connect for login to comply with the new marketplace guideline I'd like to know if it is possible to use App Engine's MailService with OAuth2 / service accounts to be able to send emails on behalf of the user.
  I know that it is possible to send emails using the old GMail API, but then we are stuck with the daily GMail limits which are far too low for our usecase.  </p>