Type:        top 1 0        and get the header of message 1.

Type:        top 2 5        and get the header and first five lines of message 2.

Type:        retr 3          and get all of message 3.

Thanks to Leonard Kostyak Jr. for the information below.

The POP3 Session - Condensed from RFC1725

*        POP3 server listens on TCP port 110
*        Client establishes TCP connection with POP3 Server by sending to
POP3 address, port 110.
*        Client is authenticated by POP3 server.
*        Transactions are performed.
*        Session is terminated and updates are made if session is
properly terminated.

POP3 Session States

        1. Authorization State
                a. POP3 server issues a greeting
                b. Client identifies self with user identification and
password with user and pass commands.
                c. If authentication is successful an exclusive
access-lock is acquired to prevent messages from being updated or
removed before update state is entered.
                d. If client is authenticated the POP3 session enters
the Transaction State.
        2. Transaction State
                a. Client may now issue any of the following commands
                        STAT - Responds with number of messages in
maildrop and size of mail drop in octets.
                        LIST msg - If an argument is entered information
on that message is returned. If no argument is entered then information
on all messages are listed. Useful if you need to know number and size
of messages in mail drop.
                        RETR msg - Message is listed.
                        DELE msg - Message is marked for deletion.
Remember to use the QUIT command when finished so messages are deleted.
                        NOOP - Does nothing, simply returns a positive
response.
                        RSET - If messages have been marked for deletion
they are unmarked.
                        TOP msg n - Returns header information for
message and returns the number of lines of the message body represented
by n.
                        UIDL msg - Returns message number and a Unique
message ID Listing.
        3. Update State
                a. Occurs when the QUIT command is issued. If the POP3
session is terminated for other reasons no update takes place and
messages marked for deletion are not deleted.


Note: This information is condensed from RFC1725. This is only a general
description of a POP3 session and was not meant to be all-inclusive. See
RFC1725 for additional information.