Windows Messenger 5.1 can receive message but not to send



I have a problem with Windows Messenger 5.1 that runs on Windows 2000 SP4
that can receive message but not to send.
The problem exists on internal network that has Checkpoint firewall ver NG
HF13 as VPN gateway(All the massages traffic is over VPN).
The firewall logs show nothing.

Please advice ?


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