[Spambayes] Beyond Spambayes

Allen netsecurity at sound-by-design.com
Sat Feb 25 07:04:03 CET 2006



Seth Goodman wrote:
> On Thursday, February 23, 2006 3:01 PM -0600,
> netsecurity at sound-by-design.com wrote:
> 
>> From this point of view it is my opinion that we'd all be better
>> off if the mail did not get delivered in the first place. This is
>> why I suggest that adding an additional layer of robustness might
>> prove useful.
> 
> I agree with you, but with a caveat.  If you run a SMTP server (incoming
> MTA), the responsibility you assume for every message you accept is that
> you will either deliver it to the final recipient or send a notice to
> the original sender explaining why it couldn't be delivered.  Since spam
> has forged return addresses, it is not possible to send non-delivery
> notifications.  Silently discarding mail that you've accepted breaks the
> assumptions behind SMTP and makes email less reliable for all of us.
> 
> The answer is to reject, i.e. not accept in the first place, spam
> messages during the SMTP transaction.  It is then the problem of the
> sending system as to what to do with the undeliverable message.  If
> stuck with enough of these, they may actually be forced to clean up
> their act as to what _they_ accept for delivery.  In any case, the key
> is in refusing to accept unwanted mail from the sending system, not
> accepting and then silently discarding it.

If routers on the backbone deleted messages with forged headers, then 
there would be a lot less for mail servers to do. Are router obligated 
to pass along all packets, both forged and malicious?

> While an individual user with Spambayes can glance at their spam folder
> and decide what to delete, nobody gets that chance when the server
> silently discards messages, and the sender isn't notified.  In that case
> the system has simply failed and no one can find out why.  Since the
> classifier that can avoid false positives has yet to be invented, it is
> very important to not silently delete suspected spam, especially at the
> server level.

The second point is about the obligation to deliver the mail regardless. 
  To use the snail mail parallel, they have no obligation to deliver 
letter bombs, why should email servers be any different? Spam or 
"legitimate" mail that carries a hostile payload is in the same category 
as letter bombs. Dump 'em in a bucket of water then dispose of them in a 
way that does no harm.

If you really wanted absolute delivery of every bit of mail regardless 
of source or content, then what do you do about routers along the way 
that get overloaded and dump masses of packets?

According to some figures I've seen (I'm not sure I believe them myself) 
about 30% of all e-mail is never delivered. While I wouldn't put the 
figure this high I do know about 2-3% of my mail is never delivered and 
I'd say the incoming loss is in the same range.

Best,

Allen


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