Another bunch of reasons to avoid OS X’s built-in mail, apart from general bugginess. I tried, (along with Calendar), but it’s just not complete. iOS has it’s own set of privacy quirks as well.
“Since the amount of spam I’m receiving appears to increase every day, I wanted to look a bit into how sensitive Mail.app is to letting spammers known I (accidentally) read their email. Turns out it is absolutely horrendous. I’m talking Outlook Express in 1995-bad. I already had a hunch that it would automatically load in external images, but it also loads <iframe>, <object>, and believe it or not, it follows redirects and loads in external websites.”