Filter message by reverse domain lookup

Hello all,
I am kinda new to Exchange and was wondering if there was a function
built into Exchange 2000 that would "look" at the source domain of the
email message and then perform a "reverse" DNS lookup to verify that
the domain actually exists and if it doesn't then have Exchange
discard the email without sending an NDR?  Is this possible strait out
of the box?  It seems like I remember iMail gateway having this
feature.  I am trying to filter out some of the spam that fills up my
users inboxes.

Thanks in advance.

Jeremy
0
jsteger (1)
11/14/2003 9:05:29 PM
exchange.admin 57650 articles. 2 followers. Follow

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jsteger@bellsouth.net (Jeremy Steger) wrote in 
news:c6e99633.0311141305.63a63722@posting.google.com:

> Hello all,
> I am kinda new to Exchange and was wondering if there was a function
> built into Exchange 2000 that would "look" at the source domain of the
> email message and then perform a "reverse" DNS lookup to verify that
> the domain actually exists and if it doesn't then have Exchange
> discard the email without sending an NDR?  Is this possible strait out
> of the box?  It seems like I remember iMail gateway having this
> feature.  I am trying to filter out some of the spam that fills up my
> users inboxes.

AFAIK you can't do anything with 2000 itself, ORF from Vamsoft will do 
this, so will Xwall and a load of free *nix MTAs.

If you are going to do this you might want to look at something that will 
reject on a temporary (4xx) basis rather than permanant - sometimes you get 
transient DNS failures and if you bounce mail based on reverse DNS you 
*will* get legitimate mail blocked.

Looking at my logs since last sunday, this is the amount of mail I've 
blocked, and on what criteria:

Sender address rejected: Domain not found (total: 2853)
User Unknown: (total: 6950)
Listed on DNSBL: (total:4515)

The second check is by far the most effective as it stops the traffic 
making it all the way into Exchange before being bounced.

The DNSBL checks are only performed on mail with specific recipients as 
we've found that of our 600 or so addresses around a dozen get 95% of the 
spam.

Every week I check the "domain not found" rejections and add the worst 
offenders that are obviously spam domains and add them to a manual 
blacklist so the next time they try they get a permanant 5xx bounce and 
usually bugger off for good :-)

regards,
Paul
-- 
paul <at> spamcop.net
0
paul9912 (95)
11/15/2003 1:31:35 PM
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