A system and method are disclosed for providing an extensible email having rich, interactive browser capabilities. The system includes a service provider for receiving extensible emails from a third party platform and presenting the emails to a user's computing device. The extensible email includes URL metadata which allows a mail user agent to fetch and run code from a third party server in order to generate the extensible email. A secure sandbox ensures that the third party code does no damage to the service provider system or the user's computing system.