An additional note that must be made at this phase is: do keep in mind the LTV or the life-time value of your customer here. In certain scenarios (e.g.: subscription-oriented affiliate programs) it makes sense paying significantly higher commissions on the customer’s initial payment to the company when the latter knows that they will make much more (from the same customer) on future payments. More about it later in this text.
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.