Treat your list well. Remember that the people you're using email to communicate with have trusted you with their email and name; they deserve your respect. Just as you deserve as a chance to convert them from customers to fans and even evangelists for your brand, people who want to talk about and share your message and get involved in any way they can.
Defendants’ program consists of a series of tiered membership levels, each with a membership fee higher than the last. Defendants pay a consumer only when the consumer recruits new consumers to join the program, through commissions on the new consumers’ membership fees. Although Defendants’ program ostensibly provides business coaching that will help members build a successful business, the goal of that “coaching” is to persuade the member to purchase a higher membership tier. – Source FTC
Emails triggered by milestones, like anniversaries and birthdays, are fun to get -- who doesn't like to celebrate a special occasion? The beauty of anniversary emails, in particular, is that they don't require subscribers to input any extra data, and they can work for a variety of senders. Plus, the timeframe can be modified based on the business model.