I have reported this issue to Apple via the "submit feedback" thing, but that's such a black hole (I love how the latest version on that page is still 10.7.1). Here's my Apple Discussions post: https://discussions.apple.com/message/16402681#16402681 The summary of it is that if you're using CardDAV to store your address book and your CardDAV account's "display name" is the same as your email address, Mail.app "helpfully" auto-expands your email address into the entire address book - EVEN ON THE FROM: LINE. And the latter one doesn't even appear on the local side (not even in sent mail). So for example, if my email address is firstname.lastname@example.org and I send an email to someone else, instead of the header saying: it says (email addresses are obviously complete fiction): The only way I was even aware that this was happening was that I got a bounce message from a spam filter - after I'd sent out several other emails as well. I have no idea how many bounces have ended up getting sent to my entire address book. I have no idea how many people have RECEIVED messages including my entire address book. This is frustrating and terrible and I am so very, very upset at Apple right now for allowing this "feature" to go in, in such a destructive way.