You'll have to remove that line from iptables if that exists outside of CSF itself. First, save the existing iptables rules:
After saving the rules, grep for the OUTPUT chain rules for port 25:
/sbin/iptables-save > /root/iptables-saved
Once you find out the line number for the spurious rule, then remove it:
/sbin/iptables -n -L OUTPUT --line-number | grep 25
Please replace line# with the line number for the bad rule in the OUTPUT chain.
iptables -D OUTPUT line#
If you are unable to find the line, please paste the full contents of the OUTPUT line here for us to see it.
If it ends up that CSF has the bad rule rather than iptables, you'll need to move your existing CSF installation and install a new copy to bypass the bad configuration that appears to have been set.