Does Kmail send this info somehow and if so, how can it be turned off?
It can't, it wouldn't be RFC compliant otherwise. If you really care, install a SMTP server and perform headers rewriting.
It's not entirely true. I had this problem when using local dns. Either use iptables to MASQ LAN with external IP or use only ISP dns in /etc/resolv.conf will solve the problem. At least that's what I did.
-Larry
