E-Parcel

E-parcel is a program for sending e-mail-like messages with big attachments.

User Interface

Alice wishes to send a 700MB MPEG file to Bob. Here's how it works:

Alice launches e-parcel, attaches the file, fills in Bob's regular e-mail address, and clicks send. That's it - same as plain old e-mail.
Bob's receives a regular e-mail with an explanation of e-parcel and a tiny attachement. When Bob's open the attachment, e-parcel asks him whether he accepts delivery of Alice's 700MB video (and a warning not to accept delivery if Bob does not have enough disk space). Bob accepts delivery. (If Bob declined delivery, Alice would be notified by e-mail, and no attachment is delivered.)
After the transmission is complete, Bob receives a second notification e-mail with a link to a file on the disk of Bob's own machine. When Bob clicks the link, the video starts playing.

Easy enough? Well, but how would you implement this?

Alice and Bob have a continuously running e-parcel daemon. They can interface with the daemon through another program, called e-parcel, that they can launch as a regular user application.
Alice launches e-parcels to send a video to Bob. E-parcels copies the file for future delivery and, meanwhile, sends an email to Bob with an e-parcel attachment (in XML) containing a request for delivery.
When Bob opens the attachment in Alice's e-mail, the e-parcel program is invoked, reads the attachment, and prompts Bob.
If Bob accepts delivery, the e-parcel daemons at Alice and Bob contact each other directly, exchange key to authenticate, authorize, and encrypt the file transmission.
The deamons transfer the files over scp and store it on the e-parcel folder in Bob's drive. When the file transfer completes, Bob is notified by regular e-mail.