Well, winpkfilter isn’t perfectly suitable to your needs, but you can work around the problems to make it work.
You can’t work with 127.0.0.1 packets, but you can simulate a “fake” host sending packets to your POP3 proxy. When the mail clients starts a connection, the PC will generate packets on the network adapter you use to connect to the internet or LAN. You can catch those packets, and send them back in. To do so, you must “simulate” a host on the same net your adapter is connected to. Suppose you are using an ethernet adapter with IP address 192.168.0.10, then outgoing packets will exit with that source IP. You must then change:
destination: POP3 IP
source: a fake IP on the same net, i.e. 192.168.0.100
destination: must be your IP on that adapter / network, in our example 192.168.0.10
Recalculate the checksums, and send the new packet back to windows, dropping the original.
Now windows should think that an external PC is sending packets, and will reply to that (fake) IP. You must catch outgoing packets with the fake destination, and send them back in reversing what you’ve done in the previous step:
destination: 192.168.0.100 (fake)
source: POP3 IP
Recalculate the checksums.