Upas - A Simpler Approach to Network Mail by David L. Presotto
“Upas 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.”
 upas,1 u´pas, n. (in full u´pas-tree´), a fabulous Javanese tree that poisoned everything for miles around; Javanese tree (Antiaris toxicara, of the mulberry family): the poison of its latex. [Malay, poison.]
These 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.