SIGAINT (sigaintevyh2rzvw.onion) is one of the largest web email services on the Tor network that is used to send messages preserving the users anonymity. The service is constantly under attack of Government agencies, in April 2015 its administrator warned the users of cyber attacks alleged launched by law enforcement agency who tried to hack the service. In 2015, a persistent attacker with access to nearly 70 bad Tor exit nodes (around 6 percent of the total) tried to compromise the email service.”]
Source: http://securityaffairs.co/wordpress/56808/deep-web/sigaint-outage-mystery.html