I think iptables handles the rules in the order they're fed to it (could be vice versa, but I think it's this way). So, following that, first create a rule that allows that one ip address have a connection to the port you specified, second create a rule that denies the IPs from connecting to it. I think that should work.
The rules would be something
/sbin/iptables -A INPUT -s 192.168.0.1 --dport 25 -j ACCEPT
/sbin/iptables -A INPUT -s 192.168.0.0/24 --dport 25 -j ACCEPT
Since I can't test them myself, I won't guarantee they'll work but that's how I'd start trying