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Mail switch naming and addressing at Cambridge - Tony Finch

dotatfanf wrote
on 5th December 2011 at 18:24
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Mail switch naming and addressing at Cambridge

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Date:2011-12-06 11:25 (UTC)
My hunch is that caching resolvers would treat them differently, though I've not worked through exactly what the differences might be. (Probably less if all the MX hosts were in the same zone as the MX record itself than if some lived elsewhere.)

The main attraction of a lower-priority backup MX has always seemed to be faster recovery after an outage: when the primary MX has recovered after a failure, the backup can be asked to flush its queue rapidly where without the backup one has to wait for countless MTAs around the world to notice you're open for business and retransmit.

However, given my mail server is currently showing a 431-day uptime and with not much care and feeding is showing at least four-nines reliability, with hindsight the backup MX might have been more trouble than it was worth.
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Date:2011-12-06 11:52 (UTC)
Your hunch is wrong. Much more relevant is the way MTAs treat them, and indeed there's no consensus on the right semantics - see section 5 of RFC 5321.

The right way to deal with outages is to have diverse locations for the MXs. Unless you are playing tricks they have to have the same accept/reject criteria, to avoid problems with backscatter, which implies they have to be basically configured the same way. The traditional third-party backup MX is generally a bad idea and has been for ten years or more.
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