I think I remember a similar leak a few years back. Wasn't it one of those Chinese or Russian botnets that got caught? Hopefully, the owners of those email addresses will change their passwords ASAP.
"Lol, 500k email addresses, that's nothing for spammers, I've seen bots scrape that many in a day. Anyone have a reliable solution for blocking spam bots from scraping emails? Maybe something with CAPTCHAs or IP blocking?"
"just found out that a popular VPN service got hacked and their user emails were leaked online. this is a huge list, just waiting for some spammer to use it to send out some malicious emails. think we'll see some phishing attempts soon."