Even if you’ve already got a long list of emails for clients and prospects, you should never stop adding to it. Especially since it’s not nearly as hard as it sounds. For example, make sure your list is always growing passively with a signup feature on your website. Subscription forms should be on your home page, blog page and everywhere else you can fit it without taking away from more important content.
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.