E-mail may seem to resemble shifting sand, but when it comes to staying power, those bits streaming through the ether might as well be carved in stone. Long after they've been forgotten, confidential strategic documents and tasteless jokes live on. And they can return to haunt the senders, as companies like Microsoft have learned all too painfully.

Disappearing Inc., a San Francisco start-up, has developed a system that promises to make such problems vanish. The company's Disappearing Email encrypts each message and assigns it a 128-bit key -- essentially a code that "unlocks" the encryption. Then the message is sent on its way. The recipient reads the encrypted mail by automatically "borrowing" the key from one of Disappearing's servers.

If Disappearing's software isn't installed on the recipient's computer, the message appears as a link to a Web site, also hosted by Disappearing. At the site the message will be joined with the key, decoded, and displayed. After a time specified by the sender, Disappearing throws away the key. The message may remain on a PC, on an E-mail server, or on backup tapes, but it would be impossible to read it.

Slated for release to companies with more than 1,000 Microsoft Outlook users, Disappearing Email is designed as an Outlook add-on. It costs $4 per mailbox per month.

Boro Marinkovich, president of BBM Solutions, a Toronto-based systems integrator, tested Disappearing Email for a client. Following a government investigation that had forced the client to turn over reams of electronic files, the company's partners were eager to try out Disappearing with their 50 or so staff members. Three months into the test, Marinkovich reported no technical problems with the service.

But Marinkovich flags possible lack of access to Disappearing's servers as his greatest concern. If Disappearing's system goes down, he points out, "you're going to have a hard time reading your mail. That's the potential Achilles' heel in this whole design."