FAKEPOP

fakepop is a POP3 server with a difference - there is never any
mail.  Any user name and password is accepted, and the mailboxes
are always empty.  It can be used during system maintenance to
keep people happy while you re-engineer your actual POP3 server.
It runs from inetd (or xinetd).

inetd:
pop3  stream  tcp  nowait  nobody  /usr/sbin/tcpd  /usr/local/bin/fakepop

Here's an example POP3 session:

>> +OK POP3 with no mail
<< CAPA
>> +OK Capabilities include
>> USER
>> PATHOLOGICAL-LYING
>> .
<< USER username
>> +OK that's a nice name
<< PASS password
>> +OK nice password, but you've got 0 messages
<< STAT
>> +OK 0 0
<< LIST
>> +OK No messages, really
>> .
<< UIDL
>> +OK No messages, really
>> .
<< RETR 1
>> -ERR you got no messages
<< TOP 1 1
>> -ERR you got no messages
<< QUIT
>> +OK cheers then


REDIRPROXY

redirproxy runs from inetd and should receive connections that
have been redirected with iptables DNAT.  If it is possible to
connect to the original destination, it will do so, and
transparently relay the connection.  If not, the configured
application is run, possibly fakepop, or the connection is
closed (tcpwrappers style).  

inetd:
pop3  stream  tcp  nowait  nobody  /usr/sbin/tcpd  /usr/local/bin/redirproxy --before /usr/local/logstart --after /usr/local/logend --unreachable /usr/local/bin/fakepop 

The before and after scripts may be useful for setting up and
tearing down the connection (e.g. dialout).