MX Issues

Kevin Darcy kcd at daimlerchrysler.com
Wed May 1 21:06:26 UTC 2002


A T Thomaidis wrote:

> I would like to ask what is the minimum declaration in DNS (the MX part)
> required for Sendmail to be able to work.

sendmail can work without any MX records at all, so I don't quite understand
the question.

> In general I find difficult to understand the concept of MX. Don't know why.
> Any good resources or explanations?

Well, the _DNS_and_BIND_ book from O'Reilly explains MX records well, I think.
But, if you've already tried that, let me try a slightly different approach...

Do you understand NS records? MX records are roughly parallel to NS records
inasmuch as they associate one name, denoting a resource, with another name or
set of names, denoting a server (or servers), for a specific function. For NS,
the function is "talk to these other servers to find out information about this
zone". For MX, the function is "talk to these other servers to deliver mail for
this particular domain name". Once you understand that, then about the only
other thing you need to understand is that, _unlike_ NS records, the
"preference" field in an MX record allows you to specify a particular failover
sequence (in the case of MX records with different preference values) or
load-balancing (in the case of MX records with the same preference value). In
contrast, NS records don't need preference values, because other nameservers
can make their own decisions about what NS targets are "better" than others,
based on round-trip-times.

Alternatively, do you understand SRV records? MX records are sort of a
simplified, mail-specific form of SRV records, minus the naming restrictions.


- Kevin





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