The Supreme Court yesterday tightened the constitutional requirements for notifying people who will be affected by "tax sales" of property.

The court said that posting a notice in a courthouse or publishing it in a newspaper before such a sale is not sufficient; authorities must notify those affected in person or by mail, the justices ruled, 6 to 3.

The case arose when the Mennonite Board of Missions in Elkhart County, Ind., discovered two years late that property on which it held a mortgage had been sold for nonpayment of taxes.

The county had posted notice and published it but had not notified the board personally. Justice Thurgood Marshall, reversing the Court of Appeals of Indiana, said the Constitution requires more. "Notice by mail or other means as certain to ensure actual notice is a minimum constitutional precondition to a proceeding which will adversely affect the liberty or property interests of any party," he wrote in Mennonite Board of Missions vs. Adams.

Justice Sandra Day O'Connor, joined by Justices Lewis F. Powell Jr. and William H. Rehnquist, dissented. O'Connor said the court was departing "significantly" from prior decisions. The board "could have safeguarded its interest with a minimum amount of effort" by checking the courthouse notices, she said. The Constitution "does not require that the state save a party from its own lack of care," O'Connor added.

In other action:

* The court upheld the Postal Rate Commission's approach to rate setting from a challenge by the National Association of Greeting Card Publishers. The decision gives the commission flexibility to determine rates without being strictly bound by the "cost of service" for each class of mail.

The ruling in National Association of Greeting Card Publishers vs. United States Postal Service was unanimous.

* The court upheld Georgia's death penalty law and reinstated the death sentence imposed on a Georgia man convicted of murder after being caught in the act of a burglary.

The 7-to-2 decision in Zant vs. Stephens approved Georgia's technique of allowing juries to impose the death sentence when they find the presence of any one of a number of aggravating circumstances.