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Exchange 2010 cannot send to Exchange 2007 in same organization (Coexsistence)

Dear expert

i've problem with sending mail from exchange 2010 (NLB and DAG) to Exchange 2007 (legacy) on same domain, same forest and same organization.

i've error that sound

"451.4.4.0 Primary target IP address responded with: "451.5.7.3 cannot achieve Exchange Server authentication. " Attempted failover to alternate host, but that did not succeed, either there are no alternate hosts, or delivery failed to all alternate hosts"

i've create receive connector on both exchange server 2010 and 2007 with authentication "exchange server" and Permission "legacy exchange Server, Exchange Server"

But still no luck ,

does anyone can help me please

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Try and restart the DNS Server service under Organization Configuration > Send Connectors > (your Send Connector name) > Properties > Network Tab > Check the box for "Use the Externam DNS Lookup settings on the transport server" Does that seem to help?
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seems that ip address looping on some receive connector<br />old admin put network address on receive connector<br /><br />after remove it. sending email from Ex2k10 to legacy normall.<br /><br />thanks