Granted that sending an email implies an informal agreement. But this agreement has to be thought of as giving exactly the rights needed to facilitate the email

Granted that sending an email implies an informal agreement. But this agreement has to be thought of as giving exactly the rights needed to facilitate the email communication, and nothing more. That means: - copyright is still with the sender (or their employer) - no rights to publish the message. The users of your service will regularly violate copyrights if they didn't get explicit permission to publish.