Не принимается почта по протоколу POP3.

Доброе время суток. Стоит Gentoo Linux c ядром 2.6.39-r3. Настроен NAT, включен форвардинг в ядре:

root@Sibox login # cat /proc/sys/net/ipv4/ip_forward
1

Сделан такой конфиг iptables:

# Generated by iptables-save v1.4.12.1 on Tue Nov 13 21:54:15 2012
*mangle
:PREROUTING ACCEPT [1961:177241]
:INPUT ACCEPT [743:68261]
:FORWARD ACCEPT [999:96596]
:OUTPUT ACCEPT [453:46394]
:POSTROUTING ACCEPT [1452:142990]
COMMIT
# Completed on Tue Nov 13 21:54:15 2012
# Generated by iptables-save v1.4.12.1 on Tue Nov 13 21:54:15 2012
*raw
:PREROUTING ACCEPT [1961:177241]
:OUTPUT ACCEPT [453:46394]
COMMIT
# Completed on Tue Nov 13 21:54:15 2012
# Generated by iptables-save v1.4.12.1 on Tue Nov 13 21:54:15 2012
*nat
:PREROUTING ACCEPT [24:1284]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
:CLIENTBANK - [0:0]
[0:0] -A POSTROUTING -s 192.168.137.0/24 -j MASQUERADE
COMMIT
# Completed on Tue Nov 13 21:54:15 2012
# Generated by iptables-save v1.4.12.1 on Tue Nov 13 21:54:15 2012
*filter
:INPUT DROP [45:14185]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [453:46394]
:CLIENTBANK - [0:0]
[560:41504] -A INPUT -p tcp -m tcp --dport 22 -j ACCEPT
[0:0] -A INPUT -d myip/32 -p tcp -m tcp --dport 3389 -j ACCEPT
[0:0] -A INPUT -p udp -m udp --dport 1194 -m comment --comment "openvpn server" -j ACCEPT
[0:0] -A INPUT -s 127.0.0.1/32 -d 127.0.0.1/32 -m comment --comment "Vajno loopback" -j ACCEPT
[1:61] -A INPUT -d 192.168.137.1/32 -j ACCEPT
[0:0] -A INPUT -p udp -m udp --dport 1194 -m comment --comment "Virtual connection server" -j ACCEPT
[0:0] -A INPUT -p tcp -m multiport --dports 25,143,110,993 -m comment --comment Mail -j ACCEPT
[1:229] -A INPUT -s 192.168.137.0/24 -j ACCEPT
[136:12282] -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
[3:144] -A FORWARD -p tcp -m multiport --ports 1024,80,21,80,9080,9443,25,110,143,993,995,587,80 -j CLIENTBANK
[0:0] -A FORWARD -i eth0 -p icmp -j ACCEPT
[0:0] -A FORWARD -i eth0 -p tcp -m multiport --ports 3389,389 -j ACCEPT
[0:0] -A FORWARD -i tun+ -p tcp -m multiport --ports 3389,389 -j ACCEPT
[0:0] -A FORWARD -p tcp -m tcp --dport 445 -j DROP
[0:0] -A FORWARD -s 10.20.11.0/24 -p tcp -m tcp --dport 3389 -m comment --comment "RDP for openvpn users" -j ACCEPT
[996:96452] -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
[3:144] -A FORWARD -s 192.168.137.0/24 -p tcp -m multiport --dports 21,20,22,23,25,110,443,995,2802,3389,5190,8108,993 -j ACCEPT
[0:0] -A FORWARD -s 192.168.137.0/24 -p udp -m multiport --dports 20,21,22,23,25,87,110,443,995,2802,3389,5190,8108 -j ACCEPT
[0:0] -A FORWARD -s 192.168.137.0/24 -j DROP
[0:0] -A FORWARD -m state --state INVALID -j DROP
COMMIT
# Completed on Tue Nov 13 21:54:15 2012

Суть проблемы в том что почтовые клиенты из внутренней сети не могут принять почту по протоколу POP3. Клиент (Outlook Express) видит, что на сервере есть новое письмо, подключается, проходит авторизацию и начинает приём письма. Вот на этом шаге всё и заканчивается. Отправка работает нормально.
Подскажите где ошибка.