Because we got blacklisted on CBL I set up the following firewall rules with iptables:

#!/bin/bash
iptables -A OUTPUT -d 127.0.0.1 -p tcp -m tcp --dport 25 -j ACCEPT
iptables -A OUTPUT -p tcp -m tcp --dport 25 -m owner --gid-owner mail -j ACCEPT
iptables -A OUTPUT -p tcp -m tcp --dport 25 -m owner --uid-owner root -j ACCEPT
iptables -A OUTPUT -p tcp -m tcp --dport 25 -m owner --uid-owner Debian-exim -j ACCEPT
iptables -A OUTPUT -p tcp -m limit --limit 15/minute -m tcp --dport 25 -j LOG --log-prefix "LOCAL_DROPPED_SPAM"
iptables -A OUTPUT -p tcp -m tcp --dport 25 -j REJECT --reject-with icmp-port-unreachable

I'm not able to connect to port 25 from localhost with another user than root or a mail group member -> So it seems to work.

Still some questions remain:

• How effective do you rate this rule-set to prevent spam coming from bad PHP-Scripts hosted on the server?
• Is there a way to block port 25 and 587 within the same statement?
• Is the usage of /usr/sbin/sendmail also limited or blocked by this rule-set?
• Is there a way to log the username of all other attempts which try to deliver stuff to port 25?