https://gigaom.com/2014/08/04/google-catches-pedophile-through-his-gmail-raising-questions-about-scope-of-scanning/
“Sadly all Internet companies have to deal with child sexual abuse. It’s why Google actively removes illegal imagery from our services — including search and Gmail — and immediately reports abuse to NCMEC. This evidence is regularly used to convict criminals. Each child sexual abuse image is given a unique digital fingerprint which enables our systems to identify those pictures, including in Gmail. It is important to remember that we only use this technology to identify child sexual abuse imagery, not other email content that could be associated with criminal activity (for example using email to plot a burglary).”