The Postal Service yesterday published detailed proposed regulations defining when it is permissible to detain or open mail.

The agency said the need for such rules "has been suggested by the recent disclosure of certain governmental mail interception and opening projects."

Congressional investigators and a commisson headed by then-Vice President Rockefeller disclosed in 1975 and 1976 that intelligence agencies, principally the Central Intelligence Agency, had for decades maintained extensive secret mail survillance projects.

In June, 1975, William J. Cotter, then chief of the Postal Inspection Service, acknowledged that there had been a CIA surveillance project until 1973, and that he had known of it two years earlier when he had denied its existence.

In a five-page notice in yesterday's Federal Register, the Postal Service refers to interception or opening of mail "by U.S. foreign intelligence agencies such as the CIA" that was conducted without a search warrant issued by a federal court.

The proposed regulations would prohibit interception or opening of mail by these agencies without a federal court warrant. "There is no statutory authority for such mail interception or opening programs," the notice says.

The notice cited executive order, issued by President Ford in February, 1976, that has been interpreted by the Justice Department as withdrawing "any prior authorization for CIA mail opening programs."

In a lengthy report Jan. 14, the department acknowledged that the mail openings conducted by the CIA from 1953-1973, involving correspondence between thousands of Americans and persons in the Soviet Union and other countries, "would be unlawful if undertaken today . . ."

But, the department said, it will not prosecute CIA employees who participated because important evidence is unavailable and because of the "state of the law that prevailed during the course of the mail opening program."

The notice covers others exceptions to the sanctity of the mails such as letter bombs or mail that violates agriculture quarantine rules. It emphasizes "the principle that mail should be generally free governmental surveillance . . ."