We are looking for a way to build a user-to-user messaging system with email relay - basically the way it works at Airbnb. We already use Twilio SendGrid. We need to get analytics on the emails like open/answer rate, time to answer, and the total number of conversations per user. Having conversations in our web app is optional at this point; the main part is the mail relay and using anonymised email addresses.

What tools can use to build the relay part? Is there a service that offers this already?

Thanks for any hints!