TCP/IP Tutorial and Technical Overview - IBM Redbooks

An encoded word can be used in the Your Email section, but not in the address
part between the < and the >. RFC 2047 specifies precisely where encoded
words can be used with reference to the syntax of RFC 2822.
15.4 Post Office Protocol (POP)
The Post Office Protocol, version 3, is a standard protocol with STD number 53.
Its status is elective, and it is described in RFC 1939. The older Post Office
Protocol version 2, defined in RFC 0937, is a historic protocol with a status of not
recommended.
The Post Office Protocol is an electronic mail protocol with both client
(sender/receiver) and server (storage) functions. POP3 supports basic functions
(download and delete) for electronic mail retrieval. More advanced functions are
supported by IMAP4 (see 15.5, “Internet Message Access Protocol (IMAP4)” on
page 591).
15.4.1 Connection states
After a POP3 client establishes a TCP connection to the server (using
well-known port 110), the interaction between the client and server passes
through three distinct states:
1. First, the POP3 server sends a greeting message to the client. Following this,
the session then enters the authentication state. During this state, the client
must authenticate itself to the server. This can be done using one of three
methods:
– USER/PASS: The combined use of a user ID and password (defined in
RFC 1939)
– APOP: Used to specify a name and an MD5 digest (also defined in RFC
1939)
– AUTH: Used to specify a mechanism (such as TLS) by which both
authentication and data protection can be provided (defined in RFC 1734)
2. If the server successfully authenticates the client, the session enters the
transaction state in which the client can access the mailbox. During this
period, the client can issue the commands listed in “Transaction state:” on
page 590.
Chapter 15. Mail applications 589