The U.S. Postal Service, whose mail delivery sometimes has been likened to the 19th Century Pony Express, yesterday was accorded a quantum leap into the space age with electronic mail.

The U.S. Postal Rate Commission recommended in a 3-to-2 decision yesterday that the postal service enter the controversial electronic-mail field which would combine modern telecommunications message transmission systems with old-fashioned postal delivery.

However, the method approved by the commissioners granted the postal service less authority than it sought.

The recommendation was hailed by the Federal Communications Commission because it would allow several qualified telecommunications firms to link with the system rather than allow the postal service to contract with one company as postal officials suggested.

The 228,000-member National Association of Letter Carriers, however, said that the commission's recommendation "is unwieldy." Union President Vincent R. Sombrotto said that although the recommendation wouldn't affect the letter carriers directly, it would be difficult to operate if functions such as billing and marketing are taken from the postal service, as the commission recommended.

The rate commission is the legal forum for proposed changes in postal rates, fees, mail classifications or the nature of postal services. The Postal Service Board of Governors, which is expected to meet next in January, must approve the decision, reject it or remand it to the commissioners.

Postal service officials had no comment on the rate commission's decision.

The electronic mail plan would allow mailers -- usually private companies -- to transmit computerized messages to post offices which would print the message, place them in envelopes and deliver them within two days anywhere in the country. The service would be used primarily for heavy mail users whose mail is based on computerized lists, such as customers' bills.

Individual customers wouldn't use the system "unless they have a Christmas card list that boggles the mind," said Commission Chairman A. Lee Fritschler.

The rate commission recommended that it review the service after Oct. 1, 1983. Fritschler said he expected the system to become operational within 12 to 18 months, depending on how the board of governors acts.

The commission rejected the plan submitted by the postal service, which would have allowed the service to contract with one electronic-transmission firm rather than open it up to others.