Why are we discussing basic network communication protocols in this book? Are we not running advanced software? Indeed we are, but knowing one's way around the protocols cannot only assist debugging a possibly non-working system but also increases the understanding of a mail system's behavior. We will start with a rather non-technical overview of the protocols, after which we will focus on the protocol details.
In the UNIX environment, traditional mail applications did not use any network protocol at all. They have instead accessed the locally stored mailbox files directly through the file system. Typically, the inbox of each user is stored in a single file in either the
/var/mail or the