special "what's my ip" pop account

Hello,

I like to enable the allow_nets Feature
(http://wiki2.dovecot.org/PasswordDatabase/ExtraFields/AllowNets) for my customers. To help them knowing there own IP I imagine a
special mailbox/loginuser at the pop3 server.

That user could give a valid pop3 answer from a dummy pop3 server or simply throw a login error with customised answer containing the IP
information.

Has anybody done something similar or ideas to build such a system?

Thanks for ideas Andreas