From @haveibeenpwned on Twitter

New breach: Email address validation service Verifications.io left 763M email addresses exposed in an open MongoDB instance, many of which also had accompanying gender, DOB and other personal attributes. 66% were already in @haveibeenpwned.

Check your email address now at haveibeenpwned.com

Read more details about the discovery of the breach on securitydiscovery.com