Another thing that Apple Mail does is to pick Unicode characters that don't map to ascii for quote and other punctuation characters.
If you view it in an ascii-only mail client you will see plenty of places where an ascii character could have been used but instead a visually similar code point has been chosen. Things like dashes, single and double quote characters.
Another thing that Apple Mail does is to pick Unicode characters that don't map to ascii for quote and other punctuation characters.