Wow, great, a condescending title! Fact is, I am using this. It used to be in the settings, I think, which is how I found out about it. And as others have stated, it's in a lot of mail clients. Apple Mail supports this, as does the iOS mail app, too.
"I haven't heard of this feature" != "no one knows about this feature".
"I haven't heard of this feature" != "no one knows about this feature".