It looks like the Chinese government is blocking user access to Google's Gmail through third-party email clients, which many people had been using as a work-around to bypass earlier blocking of Google services via web browser.
"The blocking began last Friday and has ignited anger and frustration among many Internet users in China," reports the New York Times' Ed Wong from China.
"Data from Google shows traffic to Gmail dropping to zero from Chinese servers."
Data from Google's Transparency Report show online traffic from China to Gmail fell precipitously on Friday and dropped to nearly zero on Saturday, although there was a tiny pickup on Monday.
Taj Meadows, a spokesman for Google Asia Pacific, said Google has checked its email service and "there's nothing technically wrong on our end."
Earl Zmijewski, vice president of data analytics at U.S.-based Internet analysis firm Dyn Research, said his tests showed that China's government had blocked Google IP addresses in Hong Kong used by people on the mainland to access Gmail services.
Calls to the government regulator, the China Internet Information Office, were unanswered Monday. Foreign Ministry spokeswoman Hua Chunying said she did not know about any block but that China welcomes foreign investors who conduct business legally in the country.