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mars 25, 2008
Bernstein's comment about third-party DNS and the reality with qmail
In a document written a couple years ago, Dan Bernstein describes the costs and benefits of third-party DNS services.
In his document, he states that:
``Third-party DNS means the difference between email being requeued and email bouncing.'' No, it doesn't. Mail transfer agents defer delivery attempts when DNS servers are unreachable, just as they defer delivery attempts when SMTP servers are unreachable.
This is simply not true.
If no DNS servers are available, then qmail (written by Bernstein) will bounce the message immediately with a permanent error like this:
Hi. This is the qmail-send program at some.server. I'm afraid I wasn't able to deliver your message to the following addresses. This is a permanent error; I've given up. Sorry it didn't work out. <me@foo.bar>: Sorry, I couldn't find any host named foo.bar. (#5.1.2)
If the domain owners were using third party DNS servers, qmail (or any other SMTP agent) would see that the MX servers are unavailable, and generate a temporary error. If the MX servers are unavailable for several hours, the sender will receive a temporary error message, but the message will not bounce unless the MX servers are unavailable for several days.
Posted by gfk at mars 25, 2008 11:21 AM
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