You're reading an email and suddenly there it is: A lonesome J dangling in its own nook of negative space.
According to tech blogger Chris Pirillo, someone at Microsoft decided ":)" should autocorrect to a rounded smiley in another font: Wingdings.
This issue has actually been around since 2010, back when Microsoft released another version of Outlook.
If the sender sends a rich text document and/or HTML email, Microsoft subs in your typed emoticon with a Wingding. So if your computer doesn't support Wingdings, or just isn't feeling that emoticon, it comes up as a sad, eyeless little J. (And sad faces show up as Ls.)
Though he's flagged it with Microsoft several times to no avail, it's not a true bug, Pirillo writes, so long as you don't have Windows running and don't use HTML email. If you use Outlook, his post has a step-by-step guide to disabling these little monsters.
If you don't remember Wingdings, maybe this will jar your Microsoft memories:
We contacted Pirillo if the issue has changed since 2010. He responded:
Microsoft needs to fix the problem, but they don't see it as a problem.
So, that's the problem.