Networking Explained, 2nd Edition

Chapter 3: The Internet and TCP/IP 81

then enters into an authorization state in which the client identifies itself

to the server by sending the user’s username and password for the mail

account located on the server. Assuming valid authorization, the session

then enters into a transaction state in which the client directs the server

to manipulate the user’s mailbox relative to the client e-mail program’s

configurations. These can include (among others) retrieving all unread

mail, deleting messages, and sending queued messages.

As is the case with SMTP, each POP command issued by the client generates a

response by the server. Thus, the client and POP serv ...