The short answer is that the “J” in an email is actually meant to be a smiley face.

This happens a lot if you receive an email from someone who uses Outlook.  Outlook uses the Wingdings font to show a smiley face.  The letter “J” in ASCII happens to be in the same location as the Wingding smiley, so other email clients simply show it as a “J.”

Thank you Microsoft for not following industry standards!