Email was initially designed in such a way that there were no checks and balances on the data supplied. This meant that an email could be sent by someone with a malicious intent and made to look like it came from you, anyone else or even from a non-existant address. This loophole in email is often used nowadays by email viruses and spam which forge the From address.
This service lets you specify an email address and an attempt is made to verify that it actually exists and is not a randomly forged or obsolete address.