As I'm sure you've found out, this will fail on non-RFC compliant addresses that are deliverable (as detailed in recipe 6.19 of The Perl Cookbook), as well as those people from AOL who don't know their own email address (joe@aol, because everyone uses AOL, right?)

Also, with Email::Valid, you should enable the -mxcheck option and do some SMTP afterwards to verify that what you found, has an actual account on the other side.