In his networking textbook, the world-renowned computer science expert Tanenbaum says that e-mail communication is inherently peer-to-peer. Here is the paragraph from the book:
Peer-to-peer communication is often used to share music and videos. It really hit the big time around 2000 with a music sharing service called Napster that was shut down after what was probably the biggest copyright infringement case in all of recorded history (Lam and Tan, 2001; and Macedonia, 2000). Legal applications for peer-to-peer communication also exist. These include fans sharing public domain music, families sharing photos and movies, and users downloading public software packages. In fact, one of the most popular Internet applications of all, email, is inherently peer-to-peer. This form of communication is likely to grow considerably in the future.
I have come across several people claiming the opposite - e-mail is not peer-to-peer. They say that there is no direct connection between peers. Also, one of their argument was that if you bring one of the servers down, e-mail won't work anymore. However, logically, if Alice sends an e-mail to Bob, in my eyes, Alice and Bob are peers. Just as with regular (snail) mail, anybody can send an e-mail message to anybody. There is no distinction between clients and servers. Hence, logically, a simple two-party e-mail message is a message between two peers. The route the message traverses is irrelevant.
So, is e-mail peer-to-peer? If not, what did Tanenbaum mean by saying that?