http://msdn.microsoft.com/en-us/library/system.net.networkinformation.ping.aspx
http://www.webdigi.co.uk/blog/2009/how-to-check-if-an-email-address-exists-without-sending-an-email/
http://tools.email-checker.com/
http://verify-email.org/
http://www.technixupdate.com/check-whether-an-email-id-is-valid-or-not/