Communication system for sending notification messages from servers (1), connected to a first network (2), to user terminals (3), connected to a second network (4). A notification server (5) is connected to both the first network (2) and the second network (4) and comprises a database with user identifiers and corresponding user attributes. If the notification server (5) receives a first notification message from a server (1), the notification server sends a second notification message to the user on the basis of the user-specific communication-, form- and time-attributes.