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550 5.1.1 RESOLVER.ADR.RecipNotFound; not found

Current setup
Domain A - exchange 2007 SP3, two CAS/HUB servers all domains configured as internal relay and a  send connector to forward to Exchange 2010 in Domain B.
sending emails via CAS01 works without any issues
sending emails via CAS02 failes with the following error:
550 5.1.1 RESOLVER.ADR.RecipNotFound; not found

Both server are in the same AD site, same DNS settings and firewall rules. i can telnet from CAS02 to Exchange 2010 in domain B
The message tracking log shows the message fails with
Event ID - FAIL
Source - Routing

Any help would be most welcome
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LibranDragon
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Is the destination server in Domain B allowed to receive mail (Receive Connector) from both CAS01 and CAS02?
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ITISEMEA

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Hi
yes the receive connector can in Domain B can receive mail from any IP address
remove the server from the receive connect.. Apply it..

Add it back in and check if it makes a difference
i can telnet from any of the CAS01 and CAS02 servers to exchange 2010 server and vice versa
please run an nslookup and verify what IP are being returned (IPv4 only, IPv4 and IPv6)

I have had issues in the past where a CAS won't route mail to a server when using the IPv6 address..
i have noticed on CAS02 that IPv6 was enabled whereas on CAS01 it wasnt. i have changed this and its now disabled on CAS02.
Re sent the email and it failed again, not sure if i need to wait a while for the changes to tak effect

NSLookup returned the correct IP address
wait for DNS to propagate, then check..

or on Cas02,

run an ipconfig /flushdns
run nslookup
Hi

i have let DNS update and fluished the CAS. still not working. Any other suggestions
very interesting
from CAS01 i tried sending an email via Telnet and i get sender ok message
from CAS02 same email address and destination server i get an invalid address error
check the results of nslookup from Domain B?
nslookup from domain B is okay
in the message tracking it seems to get to CAS02 but then it cant find/forward to Exchange 2010 in domain B, my networking team have checked and nothing is blocking it. It seems like CAS02 doesnt even try to lookup the address
Try using a host file entry for Domain B on CAS02. (maybe DNS, although I doubt it)
thanks for  your suggestions i have solved the issue
It was a simple restart of the CAS02 that fixed the issue
Curious, did you reboot the server after you disabled IPv6?
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thanx for the points..
self fixed