Mail::Verify provides a function CheckAddress function for verifying
email addresses.  First the syntax of the email address is checked,
then it verifies that there is at least one valid MX server accepting
email for the domain.  Using the Net::DNS module and the IO::Socket
module a list of MX records (or, falling back on a hosts A record)
are checked to make sure at least one SMTP server is accepting
connections.

WWW: http://search.cpan.org/dist/Mail-Verify/