This Perl module provides routines for checking validity of email address. It makes several checks: 1. It checks the syntax of an email address. 2. It checks if there any MX records or A records for the domain part of the email address. 3. It tries to connect to an email server directly via SMTP to check if mailbox is valid. It uses a combination of MAIL and RCPT commands which simulates sending an email. It can detect bad mailboxes in many cases. WWW: http://search.cpan.org/dist/Mail-CheckUser/