Some Virginians are feeling zapped by the new ZIP-plus-four address code.

The Postal Service announced in July that it was going ahead with the nine-digit code system, designed to expedite delivery of the 84 percent of the mail that is generated by businesses. The change to longer ZIPs has received mixed reviews from the letter-writing public.

Over the past few months, all businesses and box holders in Virginia and throughout the country have been receiving "Dear Customer" letters from the Postal Service. The letters give a new ZIP-plus-four number and add, "We hope you start using it right away."

When secretary Judy Cannington receved a notice from the Virginia Education Association mail room, she memorized her Richmond office's new number (23219-3799) and for the past few months she's been using it on correspondence and giving it out over the phone.

"Sure, a lot of people asked me what that weird number was," said Cannington. "But I really thought it was getting my mail there faster."

Although postal officials praise Cannington's spirit of cooperation, they say that without the special sorting equipment the new code requires, Cannington's letters cannot be delivered any faster.

"It does no good to use it now; we don't have the automation yet," said Lou Eberhardt, a spokesman for the Postal Service. "We will not be using it to process any mail until Oct. 1, 1983."

When Cannington heard that, she said, she was so discouraged that she ripped the ZIP notice off her bulletin board and threw it in the trash can.

Postal officials say they have developed 21 million ZIP-plus-four numbers, each consisting of the orignial ZIP plus four digits, but it will be at least a year before individual households are informed of their new ZIPs. Under the new system, ZIP code directories will have 40,000 pages.

The 15 million businesses and post office box holders in the United States received their numbers earlier, say postal officials, to allow them time to plug the new code into their internal systems.

At IBM in Manassas, a spokesman said the company will soon be having the new numbers printed on its forms and stationery. IBM was assigned four different ZIPs for its 500-acre facility.

Many companies are expected to borrow computer tapes from the Postal Service to update their address lists.

Eberhardt said representatives of Northern Virginia businesses that have not yet received their new numbers may get them by calling customer relations at the Postal Service's Merrifield center, 573-0400, ext. 253.