The heads of the nation's two largest postal unions say no mail will be picked up, sorted, or delivered on July 21 if the U.S. Postal Service tries to change the contract that now gives its 600,000 rank-and-file workers three pay raises a year. Bargaining begin soon for a contract to replace the three-year agreement that expires in mid-July.

Postal officials say that changes have to be made in the COL system that guarantees postals full inflation catchup increases every six months, in addition to regular raises. During the current contract postal workers got three regular pay raises totaling $1,500. But their big gains came from linkage to inflation.

During the past 2 1/2 years, postal workers have received five COL raises worth a total of $2,933 -- almost double the amount of their regular pay raises. Postal employes are due another COL next year. It could be worth another $600 for each of the 600,000 covered workers.

Thanks to their current contract, pay for the average postal worker (now slightly over $19,800) has gone up at a compounded rate of 29.1 percent. White collar federal workers, who get flat percentage pay raises but no COL adjustment, have received raises worth about 23 percent during the same period. Their average salary now is about $21,000 per year.

Postal officials tried, unsuccessfully, to limit or "cap" COL raises in this contract, but they were overruled by an outside arbitrator. The cap would have held COL adjustments to a percentage of the increase in the consumer price index, rather than a full inflation-catchup.

Postal brass repeatedly have said they must have some kind of COL "cap" in the upcoming contract. Union officials say that would trigger a strike, for sure. Saturday night at his installation as president of the giant American Postal Workers Union, Morris K. Biller, a militant from New York, said he was extending the olive branch to the postmaster general, and planned to enter bargaining "in good faith . . . and with professionalism." a

But he said any attempt to cap the COL could trigger a strike. AFL-CIO President Lane Kirkland was present, as was Vincent Sombrotto, president of the National Association of Letter Carriers. At the NALC convention in August, Sombrotto twice warned the postal service to leave the COL raises alone, or face a strike. Sombrotto told delegates ". . . the Postmaster General has made it clear that he says there will be no COL in the next contract. Well, I tell yo and you better listen close. If I am still your president, if there is no COL in the next contract on July 20 you won't be delivering mail in this country on July 21."

Sombrotto's union is made up mostly od "outside" workers, letter carriers, drivers and the like. Biller, on Saturday night, made it plain that his union and Sombrotto's are united in keeping the COL, even if it means stopping the mail. His APWU union is composed mainly of "inside" postal workers, clerks, and the like. Waving at Sombrotto in the audience, Biller referred to Sombrotto's August speech in which he said his people would not deliver the mail if the postal service attempts to cap or eliminate the COL raises.

Biller, whose union represents clerks and "inside" employes who sort and prepare mail for delivery, said if the postal service ends the COL raises or "tries to put a cap" on them "you [the letter carriers] won't have any mail to deliver." Both leaders emphasize they are ready for give-and-take-bargaining, but both of them -- they were leaders in New York City during the big mail strike of 1971 -- have made it clear that they aren't prepared to give on the COL issues. Whether their members will go along remains to be seen, but the prospect of a mail strike this summer will present the Reagan administration with one of its first, and biggest, labor tests in 1981.