The traditional email a friend has a few flaws, namely that it is always easier to hit forward in your email client than actually finding your friends addresses and copying and pasting them into the form you have provided. Secondly many people don’t have their friend’s addresses anymore and prefer to communicate via the likes of Facebook.
A better approach is to add a share button to the bottom of you email, like the “Add This” button you can find at the bottom of this page. Your customers can then share your email with their friends via the social network of their choice. Whether it be Facebook, twitter or anything else.
Then their friends can choose to add it to their news feed, re-tweet or whatever the appropriate terminology is for each social network, creating true viral activity. Already there are a number of brands generating significant sums of revenue from this technique.