Non host resident proxies

[Magnus Sundberg <Magnus.Sundberg@dican.se>]

Dear sirs,
I have a standard firewall, almost minimal linux 
installation+netfilter+3 interfaces (internal, external and perimeter)

I want to redirect all POP3 requests back to my main server. POP3 
requests from the main server is forwarded to the internet.
Something like this.

------------          -------------------
!          !          !                 !
! Client   !--------->! router/firewall !
!          !      ----!                 !
------------      ! ->! - - - - - - - - ! ---->(Internet)
                   ! ! !                 !
                   ! ! -------------------
-------------     ! !
!           !<----- !
! Server    !--------
!           !
-------------

I can scan all pop3 traffic for viruses by this method.
I believe this setup would be correct for proxying other protocols to.
I would preferably be interested in also being able to redirect the 
protocol port too.
Is it possible to do this with the current netfilter release?
I am aware that there is good chance of unwanted loops with this setup

regards,

Magnus Sundberg