Enable Reverse DNS Lookup
If you select this option, Exchange will attempt to verify that the client's IP address matches the host/domain submitted by the client in the EHLO/HELO command. If the reverse DNS lookup is successful, the Received header will remain intact. If the verification is unsuccessful, "unverified" appears after the IP address in the Received header of the message.
To enable reverse DNS lookup
Note Because this feature verifies addresses for all incoming messages, its use can affect your Exchange server's performance. Clear the check box to disable the feature.