People using email clients like Outlook, Mozilla's Thunderbird and apps on their phone with the SMTP and IMAP email protocols, which are used to send and receive messages, around Saturday were subject to a "man-in-the-middle" (MITM) attack, said China-based GreatFire.org.
               
A MITM attack hijacks an online connection to monitor and sometimes control communications made through that channel.
               
Attacks and blocks on foreign internet services have become increasingly common with China, which operates the world's most sophisticated online censorship mechanism, known as the Great Firewall, to eliminate any signs of dissent or challenges to the ruling Communist Party.
               
Critics say China has stepped up its disruption of foreign online services like Google over the past year to create an Internet cut off from the rest of the world.
               
GreatFire.org said on Monday that China's official Cyberspace Administration of China (CAC) was likely responsible for the MITM attack on Outlook.
               
"If our accusation is correct, this new attack signals that the Chinese authorities are intent on further cracking down on communication methods that they cannot readily monitor," GreatFire.org said on its website.
               
Last month, Google's Gmail email service was shut down in China before resuming infrequent and heavily disrupted activity, forcing many Chinese users to adopt domestic email systems.