I've never understood why some emails are ending with :J? I thought it was a meme I didn't understand. It turns out it's just Outlook doing its thing

It's not just Outlook. Teams is messing that up just as well, and so did RiF on Android.

Turns out, dealing with Unicode beyond 2 bytes (i.e. anything above ASCII and the common Latin characters) is still a problem in 2023.

Some versions of outlook autocorrected a smiley to a J in the wingdings font.