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comsat & biff
The Comsat service, working alongside the `biff` notification utility, provides users with real-time notifications of new email arrivals on UNIX systems. It leverages UDP to listen for incoming mail notifications and alerts logged-in users when new messages are received, historically enabling immediate awareness of incoming communications..
The Comsat (COMmunications SATellite) daemon is a legacy UNIX system service that collaborates with the biff utility to notify logged-in users of new incoming mail. Operating over UDP port 512, it receives mail arrival notifications from the mail transfer agent (MTA) such as Sendmail when new mail is delivered. When the biff service is enabled on a user's session, Comsat locates the user's terminal device and displays a brief snippet of the new message header directly on the terminal screen.
Designed primarily for local mail delivery systems, Comsat is invoked by the MTA via the network, quickly and with minimal overhead due to its use of UDP. The notifications are simple, unauthenticated packets sent to the Comsat daemon on the localhost, which in turn looks up session details to notify the appropriate user.
With the evolution of mail access protocols like IMAP and POP3, coupled with graphical mail clients and push notification mechanisms, Comsat has largely become obsolete. However, it remains available on many UNIX systems for legacy support and historical completeness.