What are the main e-mail marketing strategies?

The two main e-mail programs are designed for two major purposes. Promos are short, to the point e-mails with flashy images. These typically encourage the viewer to take immediate action. Immediate action can be a wide variety of things such as make a purchase or download something now. The other e-mail program are newsletters which push out much more information to boost your viewers trust and increase your credibility. By offering good quality FREE information to your viewers about your niche market they will view you as the authority on the topics thus trusting you when it comes time to make a purchase. This program works on a long term basis. You can mix the two programs but keep in mind the intended goal when hitting the send button.

Who should I send my e-mail campaigns out to?

Consider every contact point with your customers as an opportunity to gain their e-mail address and permission to send them your e-mail campaigns. Offer incentives that are exclusive on your e-mail campaigns to give people a reason to sign up.
There are e-mail address lists that you can buy but I would highly recommend staying away from them, in most cases they are scams and if they are selling lists to you that means the addresses have been bombarded with other e-mail campaigns. Just stay away.

How often should I send out a campaign?

The rule of thumb is to send out an e-mail campaign at least monthly and no more than twice a week. If you wait longer than one month your target audience might forget why they signed up in the first place and if you are slamming them with daily campaigns they will most likely get fet up with them and unsubscribe.