forwarder behind a firewall not able to communicate with collector

[Pacito21]

People,

I have a few servers behind firewall that are not able to communicate with
the collector.

Event Type: Warning
Event Source: AdtAgent
Event Category: None
Event ID: 4369
Date: 5/25/2010
Time: 4:55:03 PM
User: NT AUTHORITY\NETWORK SERVICE
Computer: xxxxxx
Description:
Forwarder unsuccessfully tried to connect to the following collector(s):
xxx.xxx.xxx.xxx:51909, status: 0x2746 (TCP connect), source: registry
addresses tried:
xxx.xxx.xxx.xxx:51909

If the list of collectors is blank, then AdtAgent was unable to locate a
collector.

Common reasons for this message are:
The machine(s) listed is not online
AdtServer is not running on the machine(s) listed
AdtServer on the machine(s) listed is not listening on the specified port
TCP connectivity to the AdtServer machine is blocked by firewall, IPSec,
or other filtering mechanism
AdtServer on the machine(s) listed actively refused the connection (due to
policy or current activity load)

I have the certificate created and in place and also I'm able to telnet the
collector using port 51909. I added the following key in the registry to try
to force the collector, but didn't help:
HKLM\SOFTWARE\Policies\Microsoft\AdtAgent\Parameters\AdtServers( with my
collector name).

Does, someone, have any idea what else could be done?
Is there anything else special to make this work?

Thanks