Quick Way to Add “Email This” Button to WordPress Posts
An “Email This” button on your posts is a great way for your readers to quickly send your articles to their friends, which may also increase your readership.
Open the single.php file in your theme. Add this little bit of code and style it with CSS to match your theme. You can add a little email icon or give it a background to draw attention to the link. Place it before or after the post, wherever you want it to appear:
<a title="Send this article to a friend!" href="mailto:?subject=<?php the_title(); ?>&body=<?php the_permalink() ?>">Email this</a>
You should get something similar to this:
Source: WP Recipes