Help with Tracing an IP Address

Hey guys I've been having trouble with this guy for a while, he has been harassing me and my girlfriend about stuff I did in the past, he claims to know me but I don't know who he/she is. I have been receiving emails from him/her, and it is frustrating because every email I get, I look in the header and I can't track the IP address because it is "Multicast" I am not THAT tech savvy but i need help to know if I could trace this IP

TL:DR I need help tracking a "Multicast" IP Address (If possible)