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That's not always reliable. For example, if you do a query on my company's web server IP address, you'll just get the name of our ISP as your result.

Whois on an IP address - you mean rDNS, right? Are you sending mail from this server without fcrdns?

Most VPS providers will let you change the rDNS record for the IP.

No, rDNS And whois are different things. http://en.wikipedia.org/wiki/Whois vs reverse DNS, which you are obviously familiar with.