"Dear Postal Customer," said one, opened by Marcy Sigler, of 1080 Fifth Avenue. "To ensure accurate and timely delivery, it is very important to make sure all the mail sent to you displays the EXACT address as it appears here: 1080 Museum Mile." The letter was even accompanied by some handy address stick-on labels.

They were not particularly well-received.

Museum Mile is the name of an association of cultural institutions on Fifth Avenue from 82nd to 104th Streets, and for at least a decade, that stretch of the avenue has officially gone by the moniker Museum Mile. There are street signs that use that title.

But people used to a prestigious address are not inclined to lose it. In a random sampling of phone calls to private homes and businesses along the route, few people had actually registered the contents of the letter. Most, when told, were aghast.

"I don't know where that came from, but it's ridiculous," said Dr. David R. Coddon, who has a practice at 1031 Fifth. "Think of all the doctors you have on Fifth Avenue. My God, all the stationary, business cards and all would have to be changed."

And Ms. Sigler, who runs a travel business out of her home and is the secretary of her building's association, was incensed.

"Now, we don't mind that Fifth Avenue has the sub name Museum Mile," Ms. Sigler said, "But to ask us to change our stationary is quite absurd. And this is a prestigious address. Who wants to change that?" She added in disbelief: "I mean, can you imagine asking a cab driver to take you to 1080 Museum Mile?"

Even the Metropolitan Museum of Art, arguably the leader of the institutions that gave the avenue its moniker, had no kind words for the idea.

"Fifth Avenue is Fifth Avenue," said Harold Holzer, the chief spokesman for the museum. "It's one of the great streets of the world. We will remain 1000 Fifth Avenue."

Over at the Postal Service, a certain amount of back-pedaling was going on.

There is absolutely no reason for anyone to change their address or stationary or to not tell someone they live on Fifth Avenue," said a spokesman, Andrew Sozzi. He said the letter had been badly worded, and that the point was not to force people to use Museum Mile as their address.

"The only thing we poorly tried to convey is that if someone tries to address a letter that way, since the street has had a duel name since 1981, then it is fine," he said, "But using a Fifth Avenue address is perfectly fine and valid, too." Mr. Sozzi said that the Post Office would send another, clarifing letter to customers.