On Fri, 8 Feb 2008, Stuart Henderson wrote:

quoted text
>On 2008/02/08 11:35, Dave Anderson wrote:
>> On Fri, 8 Feb 2008, Peter N. M. Hansteen wrote:
>>
>> >Raimo Niskanen <raimo+openbsd@erix.ericsson.se> writes:
>> >
>> >> If a backscatter gets through to sendmail, and it is to an invalid
>> >> user, what is the proper thing for sendmail to do? My sendmail
>> >> most probably does the default, which I guess is to bounce the mail.
>> >
>> >yes, if you receive a message intended for a non-existing user, you
>> >most likely bounce with 'unknown user' or the equivalent.  it's the
>> >other end, where spam apparently gets delivered, that's making more
>> >noise than necessary by bouncing messages that should have simply been
>> >forwarded to /dev/null instead.
>>
>> While I agree with most of what you're saying, quietly dropping messages
>> identified as spam is _not_ the best way of handling them -- since it's
>> rarely possible to be 100% certain that a message really is spam, and
>> it's harmful to not notify the sender that a legitimate message has not
>> been delivered.
>
>If you do this, and people forward mail to your machine, or you list
>a backup MX which accepts some spam that your machine doesn't, your
>policy results in backscatter to the envelope sender address.

As I stated in the part of my message that you didn't quote, it's not a
100% solution -- but it would drastically reduce the amount of
backscatter if everyone did it.

And, IMO, some backscatter is a much less serious problem than _any_
legitimate message disappearing without a trace.

quoted text
>There's no right answer...

There's no _perfect_ answer (short of a complete redesign of SMTP), but
there are certainly ones which improve on the current state of affairs.

	Dave

-- 
Dave Anderson
<dave@daveanderson.com>