16.6M accounts from mail.ru Dump (mail.ru) were exposed in this breach. Besides email addresses, it included passwords, which makes it a high-risk breach.
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Email addressesPasswords
In September 2014, several large dumps of user accounts appeared on the Russian Bitcoin Security Forum including one with nearly 5M email addresses and passwords, predominantly on the mail.ru domain. Whilst unlikely to be the result of a direct attack against mail.ru, the credentials were confirmed by many as legitimate for other services they had subscribed to. Further data allegedly valid for mail.ru and containing email addresses and plain text passwords was added in January 2018 bringing to total to more than 16M records. The incident was also then flagged as "unverified", a concept that was introduced after the initial data load in 2014.
Source: Have I Been Pwned (CC BY 4.0)