The tax code may need reforming, but the postal service seems to be doing pretty well.

In last Tuesday's Democratic response to President Reagan's tax reform speech, House Ways and Means Chairman Dan Rostenkowski (D-Ill.) told Americans to write to Washington in support of tax reform. "Even if you can't spell Rostenkowski," he said, "put down what they used to call my father and grandfather, Rosty. Just address it to R-O-S-T-Y, Washington, D.C."

And they did.

"Some of the letters just have 'Congressman Rosty' on it," said House Postmaster Bob Rota yesterday. "He just said send it to Washington. Some of them have just got U.S.A. on it, but we all know it's Congressman Rostenkowski."

As of Friday, Rosty's staff had received 1,000 letters, most of them supportive of reform. One Ways and Means staffer, who laughingly described himself as "a committee source," said yesterday, "It must be close to 2,000 now, eyeballing it. When the staff opened the door this morning, they said, 'Good grief!' We called the Congressional Post Office Wednesday morning and said, 'Hi, guys. You may be getting these.' They're all very political and they said, 'We already notified the city post office.'

"They all have the wrong zip code, but they seem to be getting through. They're from all kinds of people. Some are the Reagan robots who write every time the president goes on and say 'Do what the president says,' and don't even mention the subject. There are some who heard Rostenkowski talk about Democratic priorities.

"The guy has always been seen as more of an insider in the House than a public figure, and in that sense he's enjoying it."

Of course, the House Post Office is used to torrents of mail. Each day between 250,000 and 500,000 letters arrive. Compared with some of the correspondence -- like messages written on napkins -- a few letters with a quirky address seem tame.

"The only one we've had like that is Udall," said Rota. "We've gotten a lot of mail that just said, 'Congressman Mo.' We've received letters for 'Congressman Rosty' for years. People know of his father. We've always gotten it to them."

Even through rain or sleet.