I got the same thing. It seems that all LE users now have the email addresses of all other LE users?
Edit: on Twitter there seems to be someone saying that their own email is the last one in the list, so it is appending emails to the body it seems… but I’m still mobile and have not been able to confirm.
Nevermind: Now that I’m at a real computer I was able to follow more links. They’re working on it: Email Address Disclosures, Preliminary Report, June 11 2016
Bugs happen, especially these kinds which can be pretty subtle. I’ve made similar errors myself in the past (albeit with a user base of only ~150) so I totally understand. Kudos to the team for addressing it quickly! 