This question came up on the DFW SBS newsgroup in relation to SBS 2003 / Exchange 2003 feature and how it should be used for SPAM.

The answer, provided by KB297412, is that this is not a SPAM management function that will reject the message if the reverse lookup (PTR) fails – instead its just a cosmetic function that puts the hostname next to the IP address in the SMTP headers to make it easier to troubleshoot issues. Practically though its useless and also results in a performance hit.

Note: Exchange 2007 server does have the SPAM handling functionality available in the Exchange 2007 Edge Server role, where it can optionally do reverse DNS lookups and apply an action based on the result (delete/reject/deliver/etc) but thats a function of the Enterprise edition.

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