Thank you bk for this very clear explanation.
As I’m running both the mail client, the POP3 server and the packet filter on the same PC, I guess that what I must do is:
– capture the outgoing packet
– change its IP and port to localhost/110
– change its MAC address to PC’s MAC address
– recalculate checksums
– send the packet to MSTCP (as stated by SerpentFly above) and not Adapter
So, since the packet originator is the mail client running on the same PC, the source MAC address it the one I should set as MAC destination, right?