I took from here ip spoofing via http headers all the headers that might spoof my mail, and the find that client-ip header can spoof my mail.

Then, i looked in the RFC that was provided, and saw that 192.168.0.0/16 is the range of private IP’s, so i simply sent the ip: 192.168.0.1 (default gateway i guess)

Flag: Ip_$po0Fing