Disappearing has developed a technology that will let users send out email with a pre-programmed life span.

The product, also called Disappearing, is available for free from this week, though only for Outlook for Windows users.

Disappearing’s Jeff Ubois said: "We're talking to Mac developers about a Mac client or two, but we don't have a firm delivery date yet."

To set a time limit on the message, a Send Disappearing icon is clicked. The plug-in encrypts the message using the 128-bit Blowfish algorithm. It accesses Disappearing's key server through a secure link, assigns a unique decryption key, and sends the message. A drop-down menu allows a time limit for the message to be set, but the default is 45 days.

The recipient needs no plug-in or software, and the mail is readable by any mail client that can read HTML, including Hotmail or Yahoo accounts. When the email is opened, the message retrieves the decryption key from Disappearing’s key server, and displays it in decrypted form.

Once the time limit has expired, the decryption key is erased from the key server, and the message rendered unreadable.

Company representatives say they're working on a Disappearing plug-in for Lotus Notes, and expect to release free plug-ins for other popular email programs, such as Outlook Express and Eudora.