>- any other mail servers on that /24 subnet xxx.xxx.xxx.000 to xxx.xxx.xxx.254

I thought IP4 adresses are only internal and hidden toutside now, checked some headers - and yupp, some include IP4 adresses.
Feels wrong, even if by itself it should not be a security risk, as NAT on the router should make that invisible.
Still - as internal IP changes not often, it might identify specific PC with its internal adress to target that pc if surfing unsafe sites - or am I missing something ?

curious
thomas