Chinese users are said to have faced a man in the middle attack on the imap implementation of Outlook.com, the former Hotmail. Chinese who checked the mail via the browser would not be affected.
The GreatFire site discovered the attack that took place over a day and a half over the weekend. Users who had set their Outlook.com address in a mail client were confronted with an error message that they could easily click away.
The man-in-the-middle attack was made possible by a false certificate, which is self-signed and thus browsers automatically find suspicious. Mail programs also give a notification, but do not advise users to disconnect.
The attack lasted a day and a half, according to GreatFire. The attack would have resembled similar attacks on Google and Yahoo, which the site attributes to the Chinese government.