Internal POP3 strange Behavior with firewall

[acon]

I have a small network conected to internet with a zentyal gateway.
I have mail server services running for my domain, but some users access pop3 mail from external servers.
From 2 weeks ago, pop3 external access does not work.
If i do a "telnet pop3.telefonica.net 110" i get no response.
If i do the same from the zentyal server command line i get:

Code: [Select]

Connected to pop3.telefonica.net.
Escape character is '^]'.
+OK POP3 PROXY server ready

I have found that refreshing the firewall configuration, without changing anything, restore the externel pop3 access for some time.

In firewall, rules for internal networks i only have 2 rules:
- all all any
- all all pop3

If i simply change the order of these rules and save firewall, i can access pop3 for some time, but after some hours, the pop3 access is lost again.

Code: [Select]

+OK POP3 PROXY server ready

Any idea how check this?