POP3 Filter

[crystal1]

Not sure if this has been done...

Has anyone created a python script that listens on the default POP3 port
for incoming mail, kills certain messages based on a criteria, and
forwards the output to a non-standard port the POP3 server is listening on?

Upon recieving requests from a sender, the script would transparently
forward traffic to the off port unless a violating condition is encountered.

I'd like to use the listening python script to check for a correct 'to:'
  address format, attachment size limit, etc.

Before I waste too much time, do you think this is do-able?