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).

redirproxy is packaged with fakepop and also runs from inetd (or xinetd). redirproxy 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. A script can be run when the connection is initiated, and when the connection is shut down. If the connection cannot be established, redirproxy runs the configured application, e.g. fakepop.

Audience: Linux system administrators
Requires: Linux and not much else
Optional: Netfilter (iptables) for redirproxy.
Language: C
Licence: GPL

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Things to get ... If you (also) use this program, let me know (or even better, put some cash in my account...

Configuration

Here's a config file for running fakepop from xinetd - /etc/xinetd.d/fakepop: 
service fakepop3
{
    disable         = yes 
    socket_type     = stream
    protocol        = tcp
    wait            = no
    user            = nobody
    server          = /usr/local/bin/fakepop
    type            = UNLISTED
    port            = 110
}