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.