If you select this option, Microsoft SMTP Service can verify that the IP address for the domain in the From line matches the originating IP address noted in the header. This address confirms that the message originated from the computer and the domain listed in the From field. If the reverse DNS lookup is successful, the domain name for the IP address is inserted in the Received header field. If the process is unsuccessful, only the IP address is included.
Note Because this feature verifies addresses for all incoming messages, its use could affect Microsoft SMTP Service performance.