Knut Urdalen has posted a new blog item about something he calls "washing emails":

In this tutorial I’ll show you how to create a simple PHP script to cleanup a list of email addresses. As a web developer you have probably been asked to wash a list of emails from a manager or marketer some times. Here’s the ultimate solution.

His script does a few things - removes duplicates, validates that the email address exists, uses pipes for communication and is as flexible as possible to work on most PHP distributions. You can download the simple script here.