Okay, it's official: E-mail has arrived. In just a few years, it has become a preferred means of business communication. More than 160 million personal e-mail accounts will be active in the year 2000, according to the technology consulting firm Giga International. By other estimates, corporate e-mail boxes are likely to quadruple to more than 1 billion in the next two years.

But despite the near-ubiquity of e-mail, most people don't entrust sensitive business documents to it. If, say, a lawyer in Washington wants to send a copy of a will to a client in Los Angeles, the preferred means of transport is usually certified or overnight mail. Too much can go awry by e-mail -- a message can be lost, missent or, most ominously, intercepted.

That, however, might be changing, thanks to newly available "certified electronic delivery services" on the Web, such as UPS Document Exchange and CertifiedMail.com.

"Think of this as the digital equivalent of certified mail" of the U.S. Postal Service variety, said Mark Pastore, vice president of corporate development at Tumbleweed Communications Corp., an early leader in the fledgling sector. Tumbleweed, of Redwood City, Calif., provides services that underlie the United Parcel Service offering.

While many e-mail systems have "message received" functions by which senders are notified when their e-mail has arrived, this generally will work only if both e-mail parties are in the same organization, or use the same software. The new services allow any two parties to exchange documents with confirmation and assured confidentiality.

You can expect the courts to take a while working out whether electronic confirmation of delivery is as good as the piece of paper you get from the Postal Service when a registered letter reaches its destination.

Here's how the Tumbleweed system works, using the example of the aforementioned Washington lawyer. It's free to sign up for the UPS service, and it costs from 65 cents to $2.50 per e-mail, depending on what security functions are desired.

1. Signing on: The Washington lawyer logs on to www.exchange.ups.com and types in an account name and password. If the password is authenticated, the user will be connected to a secure UPS computer called a server, located in the Atlanta area. (There's also one in New Jersey.)

2. Uploading the file: With a few commands, the lawyer uploads the file (the will) in encrypted form to the Atlanta server and gives an e-mail address for the recipient.

3. Storage and tracking: The will is stored on the server, accessible only to the sender and recipient. If desired, the lawyer can take back the file before it is read. (How many times have you wished you could do this?)

4. Retrieval: The client in Los Angeles receives e-mail from the server in Atlanta saying that the file is there waiting to be picked up. The e-mail includes a special Web address that is specific to the document. The client clicks on it, is taken to the site and, if the lawyer wants, is asked to log in a special password -- usually this has been established by the two parties beforehand. This gives the client access to the Atlanta server where the will resides. The recipient then downloads the file onto a personal computer.

5. Confirmation: The server in Atlanta generates an e-mail confirmation notice to the lawyer in Washington.