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I am using Qmail and when I am sending mails I am sometimes getting the bellow error in my log for a particular domain.

Jun 22 05:20:47 221832-web1 qmail: 1308738047.397431 delivery 1040373: deferral: CNAME_lookup_failed_temporarily._(#4.4.3)/

Can anyone please tell me what are the factors responsible for such error ?

What are the DNS requests sent by Qmail when sending an email ?

FYI: The patch for the DNS cache size issue is already included in Qmail.

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Where are you seeing this error? In your qmail logs, or in your DNS logs? Can you include the entire log message in your question? – Flimzy Jun 22 '11 at 9:57
@Flimzy, I am getting this error in my qmail log. I have edit the post to include the entire error message. – Supratik Jun 22 '11 at 10:23

The answer to this is in the qmail FAQ document that came in the package. It's question 2.5, telling you that a temporary deferral is exactly that — only temporary and merely a deferral.

Sometimes this error repeats itself indefinitely until the deferred mail times out of the queue and bounces as unsendable. This is the result of a qmail bodge to work around a common software bug in the 1990s and an incorrectly sized buffer.

In 1996 Dan Bernstein modified qmail to work around a bug in BIND version 4. The way that it does this workaround can cause temporary DNS lookup failures. It is possible to remove this bodge, restoring the original qmail behaviour and averting the effects of the bodge, as long as one is aware of the negative interaction with BIND version 4 and with dnscache (which doesn't cache CNAME lookups and also has to be fixed).

Another consequence of the bodge, that is mostly averted by reverting to the original qmail behaviour, is that responses from a local proxy DNS server to qmail-remote can sometimes need to be larger than the 512 octet DNS/UDP packet size. There are qmail patches to use a properly sized response buffer for DNS lookups in such use circumstances. Reverting to using CNAME queries instead of ANY queries mitigates this, however, as it is often the extraneous resource records that qmail-remote has no real interest in that are causing the responses to exceed 512 octets in the first place.

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Thanks for the details. I came to know from my DNS provider that they had a DDOS attack on their DNS servers and this is the cause of the error. It is fixed now. – Supratik Jun 24 '11 at 4:55

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