[TriLUG] sendmail & ldap > exchange howto?

[Jason Tower]

classic dictionary attack problem with mail gateways.  i've set up a few 
gateway servers that would get tens of thousands of invalid recipient 
emails per day, all of which would then have to be bounced (usually to a 
forged sender, creating yet another problem).

i know sod all about sendmail, but postfix has a verify function that 
performs this lookup -before- closing the initial smtp session.  if the 
recipient is bogus, it does a lookup first and rejects the message if 
the recipient is bogus.  it's very easy to set up.

Paul G. Szabady wrote:
> Folks,
> 
> I'm running a RHEL 4.2 server w/sendmail (8.13) as a mail gateway.  I
> currently use a mailertable entry to forward email to an exchange server
> after some basic spam, av, and rbl filters.  The problem I'm facing is all
> the spam that gets by it which is destined for ficticious users gets
> bounced back to the gateway server.  (I'm not rejecting or discarding
> spam, just marking it.)  I'm getting 350 +/- NDA bounces a day!  I believe
> I can use Sendmail's built-in ability to perform LDAP queries for valid
> users (a.k.a. smtp addresses) in the active directory / exchange
> environment.  If I understand it correctly, this query takes place during
> the smtp layer and will bounce the email for unknown users immediately
> (from the gateway/linux server, rather than the current situation which
> lets the email into the exchange store and attempt to send a NDA to the
> sender, which is now my mail gateway.
> 
> I've been looking around for a HOWTO, but can't seem to find one.  Does
> anyone have any suggestions on where I might find a simple HOWTO?  I dread
> the thought of having to dig into the bowels of ldap to figure it if
> someone's already documented it.  ;(
> 
> Thanks in advance!
>