Tweetmeme and Facebook Share widgets shows the number of re-tweet and facebook sharing done for a particular post / article on a blog or site. You can add similar kind of easy to share button for emailing purpose and show that number as well.
Mailcounter does provide you badges to share your posts easily thru email and it will also show the number of emails done for particular post. So every time a visitor will send your post to someone thru email, this counter will automatically update and how the updated number.
Try the below button attached below to send this post thru email. It will open Outlook on your computer and create a predefined email with link of this post.
Email This Badge