Restricting your communications with customers to email can be dangerous to the size of your audience. To keep up with everyone, you should provide a benefit for providing a physical mailing address (pocket calendar at Christmas time?), and then you should mail at least twice a year. For broken email addresses, Toyon Books created a fabulous "You Bounced!" postcard. I suspect this type of notice to become an industry best practice. This card is particularly well designed–have a look! (from the VerticalResponse Email Marketing Blog).