This was only a brief mystery to me - my Outlook Reading Pane rendered it as a J, while opening the email showed the smiley face.
I only hope the people who send smileys in corporate emails one day learn that they're usually not received at the other end. Then they might STOP DOING IT!
Funnier still are the people who assume they can "recall" their email once it's sent (a feature that does work on Exchange, but obviously not in SMTP). At one previous employer, HR accidentally CC'd me on a mail suggesting my salary was out of the official comp bands. Recall! Recall! Recall!