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We have little bit biger then sand box firm, where admin whant to drive on Windows Kerio mail server.

Problem is next: firm have unstable provider abd sometimes internet is down.

Step by step:
1. We have normal working, Kerio resolve MX via localhost DNS service and all is Ok. DNS is cashed all data - it's normal too.
For example:
delivery to one recipient address:
foo<_a.t_>, DNS cached MX -> 5

2. Provider is goes down. Client send to local server mail to this recipient. Kerio receive MX from cashed DNS, begin delivery to him, but we do not have channel.

3. Kerio make fail reply from Mail Delivery Subsystem [postmaster<_a.t_>]:
<recipient<_a.t_>> ( Host not found)


CAN anybody help fix this ugly situation?!

In log we have next:
[12/Dec/2007 14:14:59] Recv: Queue-ID: 475fd0c3-000272e3, Service: SMTP, From: <my_name at>, To: <recipient>, Size: 6279, Sender-Host:, User: blya-blya-blya, SSL: yes
[12/Dec/2007 14:16:12] Sent: Queue-ID: 475fd0c3-000272e3, Recipient: <recipient>, Result: failed, Status: 5.4.4 Host not found
Pavel Dobry (Kerio)

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The problem is that Kerio DOES NOT receive MX (or A) record from your DNS server. In fact, it does receive a response from your DNS server that such domain does not exist.

It is not possible to get any IP address of receiving mailserver therefore it ends with permanent error. Sender will get a warning that the email could not be delivered. Nothing is lost.

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When we use nslookup for host we can see: internet address = internet address = internet address = internet address =

and in reply DSN we have

<> ( Host not found)
<> ( Host not found)


In this case we can see, that DNS server have MX-s, but can't resolve they - requested domain name and MX name not equal. This temporary troubles (4XX), not 5XX.

Continue reading RFC 2821, section 5: If one or more MX RRs are found for a given name, SMTP systems MUST NOT utilize any A RRs associated with that name unless they are located using the MX RRs; the "implicit MX" rule above applies only if there are no MX records present. If MX records are present, but none of them are usable, this situation MUST be reported as an error.

As i undestand the reason of error can be a windows DNS server, because they must know why they can't resolve - not reply all root servers or link is down?

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The issue lies in the DNS server returning Host Not Found when it fails to connect to the root servers / forwarding server(s). Cache can only help if the MX / A records have been queried within the TTL specified by the domain's NS. This can be as little as 300 seconds or 5 minutes or more commonly 1800 - 3600 seconds. To confirm this is the issue, during the next outage, open your command line and type "nslookup -type=all 192.168.XXX.XXX". You can use any domain you did not visit or email recently. Replace the XXX with the ip of your internal DNS server.
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