Google has revealed the identity of a user after discovering child abuse imagery in the man's Gmail account in Houston, Texas, according to a local news report.
It alerted a child protection agency, which notified the police and the man was arrested, KHOU 11 News reported.
Google told the BBC it would not comment on individual accounts.
The arrest raises questions over the privacy of personal email and Google's role in policing the web.
Police in Houston told the local news station that Google detected explicit images of a young girl in an email being sent by John Henry Skillern. After the existence of the email was referred to them by the National Center for Missing and Exploited Children, the police obtained a search warrant and arrested the man.
The 41-year-old is a convicted sex offender. He has been charged with possessing child pornography, it was reported.
"I can't see that information, I can't see that photo, but Google can," Detective David Nettles said.
Emma Carr, the acting director of privacy lobby group Big Brother Watch, told the BBC: "With the rate that Gmail messages are scanned, and the fact that all US companies are bound by US law to report suspected child abuse, it is hardly surprising that this individual has found themselves on the wrong side of the law.
"However, Gmail users will certainly be interested to know what action Google proactively takes to monitor and analyse Gmail messages for illegal content, including details of what sorts of illegal activity may be targeted. Google must also make themselves very clear about what procedures and safeguards are in place to ensure that people are not wrongly criminalised."
The BBC understands that Google does not search Gmail accounts for other forms of illegal activity, such as pirated content.
David Drummond, the chief legal officer for Google, has previously said that Google helps fund the Internet Watch Foundation (IWF), which is tasked with "proactively identifying child abuse images that Google can then remove from our search engine".
Google works with the IWF and the National Center for Missing and Exploited Children extensively, he said, adding: "We have built technology that trawls other platforms for known images of child sex abuse. We can then quickly remove them and report their existence to the authorities."
Google automatically scans email accounts to provide ads within Gmail, which has more than 400 million users worldwide.
In April, Google updated its terms and conditions to say: "Our automated systems analyse your content (including emails) to provide you personally relevant product features, such as customised search results, tailored advertising, and spam and malware detection. This analysis occurs as the content is sent, received, and when it is stored."
This occurred after a class-action lawsuit against the company over email scanning was dismissed earlier this year. At the time, Google said that "a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties".
In April Google also stopped scanning more than 30 million Gmail accounts linked to an educational scheme following reports that the scans might have breached a US privacy law.
Facebook has also faced a similar class-action lawsuit over message scanning.