Subject: Returned Mail: User Unknown 67 • Farmers consider bats their friends because of the insects they eat. Farmers consider Sendmail their friend because it gets more college- educated people interested in subsistence farming as a career. I could go on and on, but I think you get the idea. Stay tuned for the .22 penetration test results! —Rob Subject: Returned Mail: User Unknown A mail system must perform the following relatively simple tasks each time it receives a message in order to deliver that message to the intended reciepient: 1. Figure out which part of the message is the address and which part is the body. 2. Decompose the address into two parts: a name and a host (much as the U.S. Postal System decomposes addresses into a name, a street+number, and town+state.) 3. If the destination host isn’t you, send the message to the specified host.

68 Mail 4. Otherwise, use the name to figure out which user or users the message is meant for, and put the message into the appropriate mailboxes or files. Sendmail manages to blow every step of the process. STEP 1: Figure out what is address and what is body. This is easy for humans. For example, take the following message: Date: Wed, 16 Oct 91 17:33:07 -0400 From: Thomas Lawrence thomasl@media-lab.media.mit.edu To: msgs@media.mit.edu Subject: Sidewalk obstruction The logs obstructing the sidewalk in front of the building will be used in the replacement of a collapsing manhole. They will be there for the next two to three weeks. We have no trouble figuring out that this message was sent from “Thomas Lawrence,” is meant for the “msgs” mailing list which is based at the MIT Media Lab, and that the body of the message is about some logs on the sidewalk outside the building. It’s not so easy for Unix, which manages to produce: Date: Wed, 16 Oct 91 17:29:01 -0400 From: Thomas Lawrence thomasl@media-lab.media.mit.edu Subject: Sidewalk obstruction To: msgs@media.mit.edu Cc: The@media-lab.media.mit.edu, logs.obstructing.the.sidewalk.in.front.of.the.building. will.be.used.in.the@media-lab.media.mit.edu On occasion, sendmail has been known to parse the entire body of a mes- sage (sometimes backwards!) as a list of addresses: