Secure sent message identifier

A message management service allows a user to access and manage messages from various message services. The user can access the message management service using a message management client application executing on a client device and can draft messages using the message management client application and send the messages through the different message services. The message management service can add information to messages sent using the message management client application that can be used to identify and organize the messages. A secure sent-message identifier can be added to messages sent by the message management service to reliably indicate that the messages were sent by the message management service.