You could send each mail server a VRFY command, but most mail admins turn that command off since spammers use it to harvest addresses.

I'd just send an email to each address saying "We're just check to make sure your address is correct. If you'd like to update your information or be removed from our list contact us" Obviously if you get any bounces they're an inactive address.

There should be plenty of places to find a perl mass email acript. =(