I've figured out why many of my emails contain random J's. I started noticing this in college, when friends of mine switched to Outlook 2007. They were always at the ends of lines or between sentences. I would see things like:
Hey everyone, the thing is finally done. J
I thought maybe my friends' Outlook or my Thunderbird was manging line feeds or something... like the mysterious chevron characters that you see when Apache incorrectly negotiates document encoding. It wasn't until I saw one of my own messages, converted into plain text, that I realized what was happening.
Turns out, they're smiley faces.
Outlook converts the character sequence ":)" into a smiley symbol, which is Wingdings' version of the letter J. So, when reading in plain text, you see J.