Both UPS and Pitney Bowes licensed their "reliable courier service"
from a company called "TumbleWeed Software" (www.tumbleweed.com).
In a related development, the US Post Office is experimenting
with their own service: "Post ECS".
The issues aren't just scaling, there's accounting and
confirmation ("did they pick up their message?").
Note at Tumbleweed's site:
The United States Patent and Trademark Office lawfully issued
the patent, US Patent No. 5,790,790, to Tumbleweed on August 4,
1998. Entitled "Electronic Document Delivery System In Which
Notification Of Said Electronic Document Is Sent To A Recipient
Thereof," the patent claims a document delivery system in which
a user sends an electronic document from his or her computer to
a Web-based server. E-mail notification of the delivery is provided
to the recipient, who then is able to download the document from the
server using his or her browser and a direct reference contained in
the notification. Notification of the delivery is also sent to the user.