Checking the validity of an IP address

Arin.net provide a comprehensive database of who owns which ip address. If you get an email from a company but you are not sure that its genuine. Have a look at the headers and search for the IP address here. If the IP address is not traced or is owned by someone in a different country then this should ring alarm bells.

However, it isn't always as simple as that as, for example this site is hosted on a server in the USA.