Who is your email provider? Is Yahoo involved? The black diamond indicates that you're generating a character that your font doesn't know. So, what font do you use? What encoding do you use? You might be composing using Unicode but with a non-Unicode font. Having said that, I'm at a loss as to why users get this odd thing happening, notably at the ends of sentences where some users like to use two spaces. Two spaces are two spaces, and are perfectly straightforward, so why something that is not a space is creeping in is something of a mystery.