At the very least there are three e-mails you should send to your customers.  I will add that you could use this structure at the beginning of your social media campaigns as well.  These three e-mails could easily be three social media postings that you always send out and act as a strong beginning guide.

Thank you to Amanda Day for her article on VerticalResponse.com. 

1.  Welcome E-mails:  Make sure you send welcome e-mails.  This includes any customer that converts on your site, as well as, customers that share their e-mail address with you in any way.  It's certainly convenient if this practice is automated, but if not, make sure you do it on a daily basis.  It's a good way to start your day - welcoming new customers.
It's also important to welcome people who have recently "Liked" you on facebook or otherwise connected with you on social media.  Give them a shout out for making contact with you.  If you get a wave of "likes" do a group welcome.  It's a perfectly acceptable practice.
Day points out in her article that it's a good idea to tell your new customers what they can expect from you in the future.  Tell them the things you might be sending to them and how frequently to expect them.  A little more on that below.

2. Monthly communication: It could take the form of a newsletter, which is certainly easy to do.  A newsletter also acts as a catch all, allowing you to include almost anything you want to include in your communication.  I recommend monthly communication to start because for many of us, we keep from initiating e-mail campaigns because it all seems too much work.  It's important when you implement an e-mail campaign or a social media campaign that you set realistic goals, or you won't do them. 
For social efforts, once a month is not enough, so try once a week.

3. Specials/Promotions: First rule.  Make sure it really is special.  Repeat offers will be sure to drive up your "unsubscribe" requests.  It's important to remember that it's a privilege to have your customer's e-mail address.  Don't take advantage of that by sending repeat e-mails or "specials" that aren't really specials.

There you have it.  Three simple e-mails to get you started on your e-mail campaign efforts.