One nice thing about the service is that is also scrubs your list for you. Each time you send out a mailing, it records which addresses no longer exists. You can create groups of email addresses for different uses and if you email out to more than one group, the system does not duplicate the email for people who belonged to both groups.
All in all, it was a nice find when I first ran into it a year or so ago. They also offer a free trial so you can check it out before you pay. Good luck!