26.8K accounts from Staminus (staminus.net) were exposed in this breach. Besides email addresses, it included credit cards and passwords, which makes it a high-risk breach.
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Credit cardsEmail addressesIP addressesPasswordsSupport ticketsUsernames
In March 2016, the DDoS protection service Staminus was "massively hacked" resulting in an outage of more than 20 hours and the disclosure of customer credentials (with unsalted MD5 hashes), support tickets, credit card numbers and other sensitive data. 27k unique email addresses were found in the data which was subsequently released to the public. Staminus is no longer in operation.
Source: Have I Been Pwned (CC BY 4.0)