The Email Sharing screen (Emails > Email Sharing) allows you to manage the sharing options for your email. (Note: you cannot share emails that aren't Sent or Attached).
By clicking the Shared checkbox against an email, you can turn its shareable status on or off.
The Actions button has an option labelled Share On Social Networks. Clicking this will open the sharing modal, allowing you to share a link to the online version of your email via Twitter, Facebook, Google+, and LinkedIn (to your timeline, groups, and more).