Why can't mail have unpost command

[chris at mimsy.UUCP]

In article <1690 at druhi.UUCP> clive at druhi.UUCP (Clive Steward) writes:
>Of course there could be an 'unmail' command.

Certainly.

>It would work (in the user's view) like 'cancel' does in netnews.  

Indubitably.

>This seems stupendously easy.  

Not so---unless, like Usenet, just about anyone, anywhere, could
cancel your mail.  Making sent mail secure, even across something
like the ARPAnet, where physical security is inforced by DARPA, is
quite difficult.  (It is not now entirely secure:  All one can
truly tell is that mail `from user at site.edu' came from `site.edu',
unless marked otherwise.  Not all implementations provide even this
much security.)  Making revoked mail reasonably secure is even
worse.  But not impossible.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris at mimsy.umd.edu