Mailflow broken - error 451 4.7.0

Question

Hi,
i have the following situation with an exchange 2010/2013 coexistence scenario:
3 Servers, 1 2008 R2 Server as DC with Exchange 2010, 1 2012 Server as DC, 1 2012 Server with Exchange 2013. The Problem is that the mailflow on the 2013 Exchange is simply not working, and I have tried all suggested solutions i could find in the last 2 days
without success. 
Test-Mailflow fails while Test-SmtpConnectivity succeeds, but 2013-mailboxes cannot receive any mail. SmtpReceive Logs are like this:
2013-04-13T16:41:12.526Z,EX01\Default Frontend EX01,08D004497C6E9CC3,25,127.0.0.1:25,127.0.0.1:61348,*,,Message or connection acked with status Retry and response 451 4.4.0 DNS query failed. The error was: DNS query failed with error ErrorRetry
2013-04-13T16:41:12.526Z,EX01\Default Frontend EX01,08D004497C6E9CC3,26,127.0.0.1:25,127.0.0.1:61348,>,451 4.7.0 Temporary server error. Please try again later. PRX2 ,
i get the same errors when i telnet from the 2010 to the 2013 server.
so it seems like a DNS related problem, but i cannot figure out whats the problem. the ex2013 has the 2 DCs as internal DNS servers and i tried all 3 possible options of the DNS lookups setting in the server properties in ECP. i can resolve all servers without
problems on any server, so i really dont understand whats the problem here...
any ideas? somebody? please  ;)
edit:
heres another snippet from the connectivity-log:
2013-04-13T23:59:44.720Z,08D006869E9DDB49,SMTP,internalproxy,>,DNS server returned ErrorRetry reported by 0.0.0.0. [Domain:Result] = EX01.xyz.priv:ErrorRetry;
2013-04-13T23:59:44.720Z,08D006869E9DDB49,SMTP,internalproxy,-,Messages: 0 Bytes: 0 (The DNS query for  'Undefined':'internalproxy':'00000000-0000-0000-0000-000000000000' failed with error : ErrorRetry)
2013-04-14T00:00:46.316Z,08D006869E9DDB4C,SMTP,internalproxy,+,Undefined 00000000-0000-0000-0000-000000000000;QueueLength=0
2013-04-14T00:01:46.404Z,08D006869E9DDB4C,SMTP,internalproxy,>,DNS server returned ErrorRetry reported by 0.0.0.0. [Domain:Result] = EX01.xyz.priv:ErrorRetry;
2013-04-14T00:01:46.404Z,08D006869E9DDB4C,SMTP,internalproxy,-,Messages: 0 Bytes: 0 (The DNS query for  'Undefined':'internalproxy':'00000000-0000-0000-0000-000000000000' failed with error : ErrorRetry)
so the dns queries for 'undefined' and 'internalproxy' are failing, but i really don't know what that means.

Answer 1

whohoo, i FINALLY figured it out - and what can i say? the cause was another hickup of the ECP, i would call it a bug but what do i know? ;)
turns out that ex2013 has huge problems when you leave the DNS lookups setting in the server properties to "All network adapters", instead you should select a specific NIC or enter the IPs of your DNS servers manually. well, this is mentioned in numerous threads
all over the forums and obviously for many people, that was the solution.
BUT...i still had no mail flow after setting the DNS servers manually and my logs still filled up with DNS errors. in the end it turned out that the setting in the ECP only configures the transport service, but NOT the the frontend transport service. you can
easily check this with get-transportservice and get-frontendtransportservice and looking at the ExternalDNS and InternalDNS settings...sure enough, for me only the Transport Service had the configuration applied that i set in the ECP.
so, i configured set-frontendtransportservice with the same DNS settings as the transport service, and finally my mailflow is working...i can hardly believe it, and i can't believe that i found literally nothing about this behavior/bug of the ECP on the net.
so, this is my solution - maybe it helps someone else :)