The Microsoft Outlook.com email service reportedly was breached by Chinese authorities, using a "man-in-the-middle" type attack, according to the GreatFire watchdog group.
MITM attacks typically rely on hijacked online connections used to monitor and control communications through an online communication - with email users relying on IMAP and SMTP to access Outlook, Mozilla Thunderbird and other apps left vulnerable.
"We suspect that the Cyberspace Administration of China, which is directly in charge of censorship... is directly responsible for the MITM attack against Outlook, and the recent related MITM attacks in China," according to the report.
China has used similar methods to breach Apple's iCloud service, Google's Gmail, and other online email and cloud storage solutions.
Microsoft confirmed the incident to the media: "We are aware of a small number of customers impacted by malicious routing to a server impersonating Outlook.com. If a customer sees a certificate warning, they should contact their service provider for assistance."