The assault on its mail systems apparently lasted around 24 hours before returning to normal. It came after Google's Gmail was blocked in the People's Republic late last year.
The China censorship watcher said:
"This attack comes within a month of the complete blocking of Gmail (which is still entirely inaccessible). Because of the similarity between this attack and previous, recent MitM attacks in China (on Google, Yahoo and Apple), we once again suspect that Lu Wei and the Cyberspace Administration of China have orchestrated this attack or have willingly allowed the attack to happen.
"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