On Wed, 19 Sep 2007, Joel Reicher wrote:
Unfortunately, the problem with server-side spam filtering (we do it too!)
is not the false negatives; it's the false positives! I've lost important
mail due to server-side spam filtering...
FWIW I am religiously opposed to content-based filtering for exactly
this reason.
I "filter" only with blacklisting and greylisting. Although they can only
be done at a boundary mail exchanger, I find it invaluable that there's
really no such thing as a false positive; if legitimate mail bounces then
it, well... bounces.
If the mail really bounces (is returned to the real sender) then the
sender can take action. But if you accept the email, filter it, and
then try to do some sort of semi bounce by looking at the header
addresses, mail will eventually be lost.
We do all filtering at our incoming mail server while the smtp
connection still is active (using sendmail milters) and thus can return
an error message to the connecting mailserver for a true bounce.
The milters used are milter-greylist, spamass-milter and clamav-milter
in that order. Greylisting stops most of the junk mail, but enough get
through to need a dedicated server to run spamassassin and clamav.
We do have false positives, but the ones I'm aware of are 1-2 each year
for over 10,000 users. Of course, not having English as your native
language really helps to avoid false positives. And of course the number
depends on your line of work. If I was at the faculty of medicine, doing
research on Viagra, I would probably hate spam filters. We whitelist
people on request if they have these kind of problems. (I have 2
whitelisted users at the moment, and one whitelisted server).
Tagging vs. rejecting (bouncing) junk mail in an interesting
discussion. On thing often forgotten in these discussions is the number
of false positives created when people browse through tagged email,
quickly deleting everything based on the Subject header (or not
looking at all).
/Per
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