The swankiest zip code in New York is about to get even more exclusive. The U.S. Postal Service has plans to announce that the affluent neighborhood now identified by the 10021 zip code — stretching between East 61st and East 80th streets, from Central Park to the East River — will be divided into three zip codes in July, leaving 10021 for roughly a third of its original area.

“Too many people” is a reason for the change, Rep. Carolyn Maloney said, adding that was also why the Upper East Side needs the Second Avenue subway. She met with a postal district manager, Robert Daruk, on Friday. Ms. Maloney said, “Pretty soon the other two numbers will be just as honored and prestigious” as 10021.

Not everyone agrees. “This is a puzzle to me,” said the co-chairwoman of Defenders of the Historic Upper East Side, Teri Slater. Ms. Slater said 10021 was widely considered “the zip code” to live in on the Upper East Side. She joked that like Gaul, it was being divided into three parts. She said the post office would have to demonstrate a real need. “I don’t think this is going to sit favorably with many people,” she said.

An Upper East Side resident and president of a co-op on East 79th Street, Theodore Siouris, said people in his neighborhood have expressed concern over no longer being in the 10021 zip code.

Council Member Jessica Lappin said, “I would imagine many feel attached to the 10021 zip code.” The neighborhood has a reputation for fierce resistance to change. In recent months it has resisted the expansion of the Whitney Museum, which now plans a new branch in Chelsea instead; raised an outcry against a proposed glass tower by Norman Foster at 980 Madison Avenue, which the Landmarks Preservation Commission ultimately rejected; and expressed concern over turning the Seventh Regiment Armory into a Park Avenue performing arts space.

Ms. Lappin said that like the 212 area code, 10021 has cachet. In 1999 there was a stir in some areas of Manhattan when the interloper 646 was introduced, 14 years after 718 entered use.

Just how wealthy are 10021 residents? According to a nonprofit organization called Public Campaign, nearly 40% of the households have incomes of $100,000 or over. According to its Color of Money project, 10021 was the top zip code for contributions nationwide to federal election campaigns in 2000 and 2002, donating $28.4 million. That amount is more than the total from the 532 zip codes nationwide with the largest percent of African Americans combined, or from the 533 zip codes nationwide with the largest percent of Latinos combined.

Ms. Maloney said the most important issue is that everyone wants to get his or her mail on time. Ms. Slater suggested stemming the flow of junk mail instead. A spokeswoman for the USPS, Pat McGovern, said the growth in the number of addresses and the volume of mail in the neighborhood are prompting the two additional zip codes. Without describing exactly where the cutoff will be, she said the middle area would remain 10021; the area to the south would be 10065 and to the north would be 10075. She said the mail for all three zip codes would still operate out of the Lenox Hill Station on East 70th Street.

The president of the East 79th Street Neighborhood Association and a longtime community leader, Betty Cooper Wallerstein, said she could understand why the zip code change would be made. She said a contributing cause was more tall buildings appearing on “each postage-stamp-size lot” on the Upper East Side. She suggested, akin to telemarketer’s “no call lists,” a federal system to eliminate all the extra mail people neither requested nor wanted.

She said her building installed a larger waste can to hold all the junk mail.

Ms. Maloney said moving mail screening from FDR Station to the main post office has helped delivery. She said also that the postal service has said it is addressing her constituents’ complaints about long lines and vending machine problems at the Peter Stuyvesant post office further downtown.

Changes have occurred previously on the East Side. The zip code 10128, which runs from East 87 to East 96th Street, was carved out of 10028 in 1983. And compared with the 20 blocks covered by the 10021 zip code, 10028 covers just a six-block sliver from East 81st through East 86th streets.