It doesn't happen with me in Thunderbird.
If it doesn't happen with you in, say, Notepad or Wordpad or MS Word, it most likely is a setting or an add-on in your e-mail program, not in Windows. But since you don't tell what e-mail program you use, it's possible to give much more suggestions than that.
I see that it doesn't happen in the browser you use in the box you used to type your post here. That also points to your e-mail program.
Just started happening this week. When I'm typing a new email and I type a letter inside two brackets, e.g. (P) or (?) or anything else, an emoticon type icon comes up instead of the (P) etc. So far I've had a camera, lips, heart, and can't remember what else as I'm too stressed. I tried changing keyboard language but that doesn't work. Can anyone help please?