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.
Research and data analysis are key here. You can’t go in blind and hope for the best. Spend as much time as you possibly can diving deep into your data, gleaning insights, getting to know your customers and your market. Interviews, surveys, and data analysis are all your friends here.