It's not a leak if it's just email addresses. Email is useless if it's not public. If you get spam or unwanted messages, then set some filters. If you can't even do that, you probably shouldn't be using email. If you can't ignore things like that, you're probably better of not using the Internet.
If email addresses are a leak, then I guess that makes the phone book a disaster. Real names matched to addresses and phone numbers, oh my!
It's vaguely possible that some email name schemes could reveal something of an organizational structure that could be of some sort of value to someone. You'd be able to get multiple addresses from the same org, most likely.
But that would indicate security flaws with the naming scheme as much as anything else.
If email addresses are a leak, then I guess that makes the phone book a disaster. Real names matched to addresses and phone numbers, oh my!