451 5.7.3 Cannot achieve Exchange Server authentication

hi

I installed 3 Exchange 2010 Server in my organization. the servers are on 3 sites conencted via VPN.
When looking at the queue on the hub, the messages are in que with message "451 4.4.0 Primary target IP address responded with: "451 5.7.3
Cannot achieve Exchange Server authentication."
On EMC  Configuration->Hub Transport->Receive Connectors->Default Connector->Permission Groups the Anonymous users and Exchange server are  enabled.

what else can i do to resolve this problem?

regards
LVL 5
deibelAsked:
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AkhaterCommented:
this is typically due to a firewall between the different sites, please make sure to disable any SMTP inspection on the firewalls
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deibelAuthor Commented:
it was the firewall
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