11.7.1 POP3 Objects 
All POP3 commands are represented by methods of the same name,
in lower-case; most return the response text sent by the server.
An POP3 instance has the following methods:
- getwelcome ()
 - 
Returns the greeting string sent by the POP3 server.
 
- user (username)
 - 
Send user command, response should indicate that a password is required.
 
- pass_ (password)
 - 
Send password, response includes message count and mailbox size.
Note: the mailbox on the server is locked until quit() is
called.
 
- apop (user, secret)
 - 
Use the more secure APOP authentication to log into the POP3 server.
 
- rpop (user)
 - 
Use RPOP authentication (similar to UNIX r-commands) to log into POP3 server.
 
- stat ()
 - 
Get mailbox status.  The result is a tuple of 2 integers:
(message count, mailbox size).
 
- list ([which])
 - 
Request message list, result is in the form
(response, ['mesg_num octets', ...]).  If which is
set, it is the message to list.
 
- retr (which)
 - 
Retrieve whole message number which.  Result is in form 
(response, ['line', ...], octets).
 
- dele (which)
 - 
Delete message number which.
 
- rset ()
 - 
Remove any deletion marks for the mailbox.
 
- noop ()
 - 
Do nothing.  Might be used as a keep-alive.
 
- quit ()
 - 
Signoff:  commit changes, unlock mailbox, drop connection.
 
- top (which, howmuch)
 - 
Retrieves the message header plus howmuch lines of the message
after the header of message number which. Result is in form 
(response, ['line', ...], octets).
 
- uidl ([which])
 - 
Return message digest (unique id) list.
If which is specified, result contains the unique id for that
message in the form 
'response mesgnum uid,
otherwise result is list (response, ['mesgnum uid', ...],
octets).
 
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