Mail-validation is a free service that allow you to check wether an email address is valid or not by connecting to the smtp port of the mail address and checking for his existence.
If you do not clear your subscriber list regularly you might end up with an high percentage of bounce due to invalid email address.
If this happen too often you will suffer consequences !
All the mail providers and delivery system have high anti-spam filter that you will trigger with such behaviour. As a consequence your email will end up in the spam box and will never be seen again by your subscriber's list.
Make sure to check if your email addresses are valid so you don't have to worry about those problems :)