(begin quote)
It's actually not particularly hard.
Server-side, if you have the server doing cyclic
redundancy checks, or hashes and storing them for each
e-mail message, then not allowing the message to come
through more than say, 4 times in total...and not more
than once to a single person - you can block mass
messages --- this being much more effective the more
users you have...
The ISP would likely also have a growing list of
blocked e-mail addresses (to known spammers) that
would automatically get deleted...and possibly some
keyword checking "big sale" or "free pron" etc.
For anti-virus, you simply have a scanner checking for
the main virii going around and auto-matically
deleting them if they're being received.
- W0rm
(end of quote)
Problem with blocking the same message coming through many times is it would
disrupt the operation of emailing lists. Also, sometimes, somebody might send
the same or nearly the same message several times, possibly by mistake. Or
maybe it was blocked at the other end by a down server. I had some comical
problems when I was using OS/2 SENDMAIL. Sometimes the message would send OK
despite error message, or would not send despite no error message. One
recipient got the same message from me seven times due to such an error;
naturally I explained why.
Keyword checking seems high-risk, especially the examples you gave. One might
simply be discussing a message he/she received about free pr0n, or may be
planning to take advantage of a big sale at some online or brick-and-mortar
store.
All this checking by the server might add substantially to the processing time.
Still, some news servers are able to intercept spam. I notice, from other
users' responses, that my news server had blocked some off-topic spams.
