China has ordered internet service providers to screen private email for political content.
New rules posted on the Ministry of Information Industry website also hold operators responsible for subversive chatroom postings.
Any writings containing "sensitive materials" must be turned over to authorities.
The rules are reported to point to Beijing's resolve to keep a firm hand on the medium, despite China's entry into the World Trade Organisation.
According to the South China Morning Post , the Ministry of Information posting included a rundown of specific banned content areas.
Outlawed writings include any that reveal state secrets, feature pornography and violence or advocate cults.
There are also measures to limit the use of foreign software. Overseas companies will have to sign contracts with an undertaking not to supply software that includes hidden programs.