J in emails represents a smiley face when the Wingdings font is not installed or if the email client does not support formatted text. Since most Windows and Mac computers have Wingdings installed, typing :) or :-) is normally automatically converted into a smiley face emoticon in Microsoft Office.
If using Microsoft Outlook as the default email client, sending Js can be prevented by setting the computer to automatically replace the Wingdings smiley face with the standard :) or :-). To do so, go to "Options," then "Mail," then "Spelling and AutoCorrect" and finally "AutoCorrect Options," where you can set an option to convert :) and :-) to ☺. This smiley is a unicode character supported by all computers.