Exchange 2010 - 451 4.4.0 DNS query failed

Hi Guys,

We just started using mimecast for spam filtering, inbound emails works fine but problem is with the outbound emails.

Basically email should be routed through the mimecast servers but when use the mimecast's server FQDN on the send connector the email gets stuck in the queue with the 451 4.4.0 DNS query failed error ( see attached ).

I thought it was DNS issue with the exchange server and for that reason I ran a nslookup but it resolved the names properly, then I tried with telnet and I was able to connect and even to sent emails through the eu-smtp-outbound-1.mimecast.com server.

This is not a firewall issue because I am able to run telnet tests.

I have been working on this for the past 2 days but still unable to figure it out :(
Queue.jpg
R2_D2Asked:
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DrDave242Commented:
Did you follow the procedure shown here for setting up the SMTP connector?
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Simon Butler (Sembee)ConsultantCommented:
Check whether you have external DNS servers listed on the Receive Connector. That can sometimes cause this problem.

Also be aware of this issue with DNS:
http://support.microsoft.com/kb/2508835

That can be caused by some firewalls.

Simon.
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R2_D2Author Commented:
DrDave242, I did follow the instrucitons from the link but didnt help.

Simon, my exchange server is win2012 and the patch was incompatible.
Also my default received connector had some IP addresses on the " receive mail from remote servers that have these ip addresses " and i just removed them and left the default settings " 0.0.0.0-255.255.255.255" and "::ffff.ffff.ffff.ffff "but unfortunately it did't fix it :(

this issue is very weird.
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skullnobrainsCommented:
in the properties of the smarthostdeliveryconnector, Network tab,

try to check "Use the External DNS Lookup settings ont the transport server"

if already checked, try to specify dns servers explicitely

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as a quick workaround for the time being, adding a line in the hosts table or using the ip addresses associated with the mimecast server in the connector should work
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R2_D2Author Commented:
tried with the "Use the External DNS Lookup settings ont the transport server" but didn't work neither.

How can i  specify dns servers explicitely ??

Finaly, I didn't edit the host file because the eu-smtp-outbound-1.mimecast.com resolves to a large range of ip addresses.
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skullnobrainsCommented:
through the properties of the "external dns servers"

you can associate as many ip addresses you want with a single name in a host file by adding multiple lines. this is only a poor man's workaround

can you confirm that running nslookup on the exchange server gives the proper address ?
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R2_D2Author Commented:
Sorry for the late reply.

The Setting "Use the External DNS Lookup settings on the transport server" and restarting the exchange server fixed the problem.

Thank you for your help.
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skullnobrainsCommented:
good to know this required a restart. thanks for sharing

best regards
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