Dubbing itself "The Nation's New Postal System," MCI Communications Corp. yesterday formally introduced its new electronic mail service that's intended to deliver both computer-based and paper-based messages nationwide at costs and speeds below those of competing delivery services.

The system, called MCI Mail and currently in operation, exploits existing "packet switching" telecommunications technology to enable owners of otherwise incompatible personal computers, word processors and computer terminals to send messages instantaneously to one another over the MCI network.

"Ours is an instant information economy," said William G. McGowan, the company's chairman, "and this is the postal system designed for it."

For intended recipients who lack a computer, MCI has an arrangement with Purolator Courier Corp. for a paper-copy message delivery.

Essentially, says MCI, its new service can reach "every deliverable address" in the United States. The company offers four delivery options. The first is "Instant" mail, which lets the customer send a 1,000-word computer-to-computer message for $1; the second is "Four Hour Service" whereby, for a $25 fee, Purolator will deliver a hard-copy message; the "Overnight" option costs $6 for five pages delivered before noon the next day; and the MCI Letter has the company place the message at the local U.S. Postal Service for delivery. In each case the sender is using a computer-like device.

The new service, McGowan said, will enable the company to compete for a lion's share of what it says is an $8 billion market for "time-sensitive" messages. "We are creating a new industry and a new institution with MCI Mail," asserted McGowan.

Modifying MCI's existing network to accommodate its electronic mail offering has cost the company roughly $40 million, and MCI said the service "would run at a loss for the first two years."

MCI is backing its new offering with a $30 million advertising campaign, comparable to its long-distance service advertising expenditures, and projects that roughly 20 million messages will be sent in MCI Mail's first year of operation.

The technological key to the electronic mail system is a telecommunications center in Naperville, Ill., that is designed to mediate between the various and incompatible brands of communicating computers and serve as the digital postmaster.

Essentially, an MCI customer would sign on to the service by dialing a local number, and would type in the name and address of the intended recipient, plus the message. The message would be sorted by the Naperville computers and dropped into the appropriate electronic mailbox.

When the intended recipient signed on to the service, he would be alerted that a message was waiting. The Naperville computer also distributes the messages to the various MCI Postal Centers around the country that tie into Puralotor or the U.S. Postal Service.

McGowan said that the mail service was targeted to consumers as well as business and that the rapid growth of personal computers with communications capabilities would enhance the service's chances of success.