The Mail Handlers Division of the Laborer's International Union and the Postal Service announced yesterday that they had reached tentative agreement on a three-year contract covering about 51,000 workers throughout the country.

But two other postal unions holding joint negotiations with the Postal Service said after the announcement that they are not close to agreement and denounced the Mail Handlers' contract as a "sellout."

The contracts with the three unions run out at midnight Monday.

Postal workers cannot walk out because of a law banning strikes against federal agencies. Three years ago, when the Postal Service and unions were unable to reach an agreement before the deadline, the contracts were submitted to binding arbitration for settlement.

Vincent R. Sombrotto, president of the National Association of Letter Carriers, and Moe Biller, president of the American Postal Workers Union, said they would not even have considered the "obscene" terms of the Mail Handlers' settlement as an opening bargaining proposal in their negotiations.

"They are going to have to make significant progress from where they are with the Mail Handlers for us to have a contract," Sombrotto said.

The Mail Handlers "have no doubt agreed to and signed the most shameful contract," Biller said, "in the 17 years of collective bargaining in the Postal Service."

The Postal Workers Union includes about 346,000 workers, and the Letter Carriers union has a membership of almost 233,000.

Negotiators for the two unions earlier yesterday submitted a proposal calling for a 6.8 percent salary increase in each of the next three years, reduction in the amount of time it takes to rise from the lowest salary to the highest, and 100 percent coverage of medical insurance and cost-of-living increases. Negotiations were scheduled to resume this morning.

The major sticking points in negotiations involving the Letter Carriers and the Postal Workers involve wage increases and the Postal Service's use of "casual," or part-time labor. The two unions have charged that the Postal Service is trying to establish a "permanent, cut-rate work force" by using nonpermanent employes.

Sombrotto and Biller said the contract the Mail Handlers agreed to included an average salary increase of 1.6 percent a year and made major concessions to the Postal Service on overtime pay. The contract also included a "me-too clause" that entitles the Mail Handlers to any benefits the other two unions gain in their ongoing negotiations, the two union presidents said.

The Postal Service would not comment on the terms, and Mail Handlers officials said in a statement they will not publicly disclose the details until members have been informed.

Louis Elesie, international trustee for the laborer's union that includes the Mail Handlers, commended Postmaster General Preston R. Tisch and said Postal Service negotiators bargained in good faith.

"Mail Handlers will be proud of this new contract," Elesie said. "It provides substantially improved economic provisions and strengthened job security protections for all workers covered by the agreement."

In a separate statement, Tisch hailed the agreement as "in the best interests of the Mail Handlers, the best interests of our customers and the best interests of a healthy national postal system."