I have setup my exchange server to perform reverse lookup on incoming messages.  The problem is when spam messages are sent and the internet header says reverse lookup failed, why did it not reject the message.  is there a registry tweak that i am missing?
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Take a look at microsoft kb article 297412.  I think you are misinterpreting what the Perform Reverse DNS Lookup for Incoming Messages option is used for.  That option is not designed to reject mail from spammers. Instead, all it does is resolve the IP Address to a host name and then attach that host name to the header.


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Exchange does not support blocking based on reverse lookups, you can enable
Exchange to force a reverse lookup, but if it fails it is only going to slow
down the connection and nothing else.

Exchange 2003 offers other ways to combat SPAM.
You can check the Intelligent Message Filter for example:

Other feature include recipient filtering where you only accept emails for
users on your local server, connection filtering where you can configure
Exchange to perform a check against one of the black lists available on the
Internet (this is managed by external entities and the servers they list may
or may not be correct), or you can check some third party products that are
available that will perform additional filtering, some of them have freeware
editions. An MVP should be able to provide you more details on that third
party software and their experiences with it.
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