Internal mailflow not working between exchange 2007 and 2010 servers.

Internal mailflow not working between exchange 2007 and 2010 servers.what needs to be check its an new migration or transition.
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Yogesh_Exchange_ExpertAuthor Commented:
below error is coming when i am doing message tracking..

The last attempt to send the message was at 10/18/2011 1:19:07 PM (UTC-07:00) Mountain Time (US & Canada) and generated the error '451 4.4.0 Primary target IP address responded with: "421 4.3.2 Service not available, closing transmission channel." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.'.
service not available is usually a routing issue

are they both in the same AD site? can you telnet from one server to the other on port 25? do you have any kind of firewall between them ?

1. Make sure you can telnet from one to the other and that you are getting the Exchange banner as reply
2. Make sure that SMTP inspection is DISABLED on the firewalls
3. make sure DNS resolution is working between the two

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