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Upas - a simpler approach to network mail by David L. Presotto
"Upas[1] is a mail interface that routes messages between existing network specific mailers, users, and user mailboxes. It uses a regular expression based mini-language to convert mail addresses into the commands needed to route the mail to the intended destination. Upas is the mail interface for the Eighth Edition of the Unix Time-Sharing System."
See Also
This days the most recent version of Upas is part of Plan 9 and includes a file system interface using 9P (upas/fs), a port back to Unix is part of Plan 9 from User Space.