It's because most registration form not only checks that the email address you entered is valid but also whether it exists. This is done by using nslookup to qu

It's because most registration form not only checks that the email address you entered is valid but also whether it exists. This is done by using nslookup to query for a domain's MX record and then checking the specific address. Here's a post I found detailing how to use it: http://www.webdigi.co.uk/blog/2009/how-to-check-if-an-email-address-exists-without-sending-an-email/ http://www.webdigi.co.uk/blog/2009/how-to-check-if-an-email-...