Let me see if I can remember this after using FreeBSD for so long. Are you using iptables? If so, try this:
iptables -t nat -A PREROUTING -p tcp --dport 23 --to-destination 192.168.0.2 -j DNAT
I hope I got that right. I couldn't remember what port mail goes on so you'll definately have to change the 23.