Two particularly important groups for our purposes are customers with a one-time purchase and customers who have purchased multiple times. A customer is sometimes not considered to be loyal or repeat customer until they have purchased two to five times, in which case the single purchase segment is more akin to a warm prospect than a loyal customer.
When you're using email as a marketing tool, you're working on a pretty personal basis. This isn't ‘drive-by' stuff -you're directly targeting customers from their inboxes. There's a huge amount of potential which comes with this kind of marketing... but you'll only reap that potential if you REALLY understand both the marketplace you're operating from, and the audience you're talking to.