Mediaboss Marketing

The watchdog called this form of attack which affects mobile devices when emails are being downloaded automatically in the background as "especially devious."

"The warning messages users receive from their email clients are much less noticeable than the warning messages delivered to modern browsers," Greatfire's report said.

Read MoreUS charges China with cyber-spying on American firms

The Microsoft spokesperson added: "If a customer sees a certificate warning, they should contact their service provider for assistance."

When a user opened their inbox on their phone, a message popped up which said the identity of the email server could not be verified. But the Greatfire report said consumers will "not think twice" before clicking the "continue" option on the error message as they would likely attribute it to a network problem.

If a user hit continue, their emails and login credentials would be in the hands of the hackers.

"We strongly recommend that users never bypass certificate error messages by clicking 'continue'," the report warned.

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Microsoft Outlook 'hacked' by China