IPTABLES filter question

Thanks in advance to any IPTABLES gurus out there who may be able to help me!

On one of my servers, running IPTABLES version 1.2.8, I could enter the following
rule to block all incoming mail from the 200.0.0.0 IP block:

iptables -A INPUT -i eth0 -p tcp -s 200.0.0.0/8 --destination-port 25 -j DROP

... and the rule set would work flawlessly, dropping incoming packets headed to this server's
port 25.

On a newer server, with a more recent version of IPTABLES (1.2.9), this syntax is
apparently not supported, as packets from 200.whatever flow into the system
unimpeded.

My clients on this particular machine have no need of incoming email from this region of South America, and have asked for a blanket filter.

I'd love to be able to oblige them, and need a bit of help with regard to the new syntax that is evidently called for with IPTABLES 1.2.9.

A big THANK YOU for any thoughts or advice.

// Bill