Faca assim:

# --- regras de ACCEPT ---
iptables -A FORWARD -p tcp -m tcp --dport 110 -j ACCEPT
iptables -A FORWARD -p tcp -m tcp --dport 25 -j ACCEPT
iptables -A FORWARD -p tcp -m tcp --dport 80 -j ACCEPT

# --- regras de DROP ---
iptables -A INPUT -p tcp --dport 21 -j DROP
iptables -A INPUT -p tcp --dport 22 -j DROP