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
ITISEMEAAsked:
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LibranDragonCommented:
Is the destination server in Domain B allowed to receive mail (Receive Connector) from both CAS01 and CAS02?
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ITISEMEAAuthor Commented:
Hi
yes the receive connector can in Domain B can receive mail from any IP address
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LibranDragonCommented:
remove the server from the receive connect.. Apply it..

Add it back in and check if it makes a difference
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ITISEMEAAuthor Commented:
i can telnet from any of the CAS01 and CAS02 servers to exchange 2010 server and vice versa
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LibranDragonCommented:
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..
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ITISEMEAAuthor Commented:
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
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LibranDragonCommented:
wait for DNS to propagate, then check..

or on Cas02,

run an ipconfig /flushdns
run nslookup
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ITISEMEAAuthor Commented:
Hi

i have let DNS update and fluished the CAS. still not working. Any other suggestions
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ITISEMEAAuthor Commented:
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
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LibranDragonCommented:
check the results of nslookup from Domain B?
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ITISEMEAAuthor Commented:
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
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LibranDragonCommented:
Try using a host file entry for Domain B on CAS02. (maybe DNS, although I doubt it)
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nashim khanExchange AdministratorCommented:
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ITISEMEAAuthor Commented:
thanks for  your suggestions i have solved the issue
It was a simple restart of the CAS02 that fixed the issue
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LibranDragonCommented:
Curious, did you reboot the server after you disabled IPv6?
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ITISEMEAAuthor Commented:
i disabled IPv6 in the morning and then he reboot happened late evening
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LibranDragonCommented:
thanx for the points..
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ITISEMEAAuthor Commented:
self fixed
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