The email database of controversial British anti-piracy firm ACS:Law was leaked on file-sharing website the Pirate Bay at the weekend.
Versions of its s emails were available for download, according to listings on the Pirate Bay, including a file marked as having been scrubbed of viruses and another sorted for easy reading.
ACS:Law was under investigation by the UK Solicitor's Regulatory Authority for its practice of mass mailing alleged file-sharers settlement offers in order to avoid litigation.
According to torrent news website Torrent Freak, 30 percent of the 11,367 letter recipients over two years had chosen to settle for between £350 and £700 ($577 to $1,154) an allegation, which netted ACS:Law over £600,000 ($989,000).
The leaked ACS:Law emails purported to include the revenue split between rights holders, software companies that monitor file-sharing and ACS:Law, as well as the success rate of the letter campaigns.
The leak allegedly occurred after ACS:Law posted a backup of its email database to its website, which it was restoring following a distributed denial of service attack.
At the time of writing, the website was offline.
Its website was forced offline last week in attacks launched by 4Chan message board pranksters under their "Operation: Payback" campaign.
It targeted the Motion Picture Association of America the international federation of phonograph industries and an Indian software firm that claimed it used such attacks against the Pirate Bay.
ACS:Law's Andrew Crossley reportedly told The Register on Wednesday - prior to the email leak - "big whoop" about the attack, adding that his site was "only down for a few hours".
By Friday "Anonymous" leaders of "Payback is a B----" were taunting Crossley with his own emails.