Insider threat or human error shows up a lot as the major cause of data breaches. But often it s not defined, or it’s not clearly defined, so people conjure up their own definition. The best way to combat this is to have clearly defined hiring, onboarding, and offboarding procedures and use a password management system. Employees should not have the ability to access anything using personal emails, and once they’ve left the company you should turn off their access completely.
Source: https://threatpost.com/dnsfilter-insider-threats-what-are-they-really/162261/