WP-Mail-Validator
WP-Mail-Validator could be configured to take a 3 step mail validation: 1. syntax 2. host 3. mx-record of mailserver.
Screenshot of WP-Mail-Validator
Plugin Name: WP-Mail-Validator
Version: 0.4
Information Link: http://blog.marcus-kimpenhaus.de/
Download Plugin: WP-Mail-Validator