From the L.A. Times‘ “Cyberspace Gives Al Qaeda Refuge“:
Simplicity seems to work best. One common method of communicating over the Internet is essentially an e-mail version of the classic dead drop.
Members of a cell are all given the same prearranged username and password for an e-mail account on an Internet service provider, or ISP, such as Hotmail or Yahoo, according to the recent joint report by the Treasury and Justice departments.
One member writes a message, but instead of sending it, he puts it in the ‘draft’ file and then logs off. Someone else can then sign onto the account using the same username and password, read the draft and then delete it.
‘Because the draft was never sent, the ISP does not retain a copy of it and there is no record of it traversing the Internet—it never went anywhere, its recipients came to it,’ the report said.