The U.S. Postal Rate Commission slightly modified its proposed plan for electronic mail yesterday in response to requests by the Postal Service Board of Governors.

By a 3 to 2 vote, the commission in its formal recomendations to the governors clarified its position on the Postal Service's authority to compete in the electronic transmission of messages by contracting with private firms.

The commission said that it recognized the Postal Service's authority to contract with firms, but it also must compete with other firms in the electronic mail system.

The electronic mail plan would allow mailers -- usually private companies -- to transmit computerized messages to post offices which would print them, place them in envelopes and deliver them within two days anywhere in the country.

The service would be primarily for heavy mail users whose mail is based on computerized lists such as customers' bills.

The Postal Service had suggested that it be allowed to contract with private firms to transmit the messages without competition from private firms.

The Postal Rate Commission then ruled that other private firms should be allowed to transmit the messages, which then would be processed by postal employes. The commission also said -- but failed to include in its recommendations -- that the Postal Service also could contract with a telecommunications firm and compete with other private businesses if it demonstrated a need to do so.

But in February the governors requested that the Postal Rate Commission specially spell out in its formal recommendations that the Postal Service be allowed to participate in message transmission by contracting with firms. That is what the commission did yesterday.

In reponse to the governor's February recommendations, the Postal Rate Commission also ruled yesterday that the system should be experimental and not permanent as the governors suggested. The commission suggested extending the program's experimental authorization from Oct. 1, 1983, to Oct. 1, 1984.

Two of the five commission members dissented, saying that the other commission members were slowing up implementation of the electronic mail system.