If you use scripts on your server, they can be misused by spammers to send messages directly over port 25 without being filtered through our outgoing filter. Actually, there is no reason why a script should be able to send messages via port 25.
To prevent outgoing connections over port 25 and to pass all traffic through the EuropeanMX filter, you can use the following IP tables in your firewall:
iptables -A OUTPUT -p tcp --dport 25 -j DROP
iptables -D OUTPUT -p tcp --dport 25 -j DROP
ip6tables -A OUTPUT -p tcp --dport 25 -j DROP
ip6tables -D OUTPUT -p tcp --dport 25 -j DROP
Please make sure that the rules are applied automatically after a restart.