Usually, websites do not give a response from the API, that a mail has been sent (the site will just display static text and each service is self-contained and should use a buffer-system, which enables you to restart the service and work on everything which has failed before). However, if you need such feedback, you could use the bus to send back an "success" message. That would require a token on the message, so that other services can recognize to what the success message belongs.
That method, however, harbors dangers, as it is tempting to wait for success, then do something and send another message out. In the worst case, you create a very hard to debug loop of messages which answer to messages which answer to messages (and so on).