If people want advice on dual-stack MX setup, see RFC3974. This
document is 3 years old, but I believe what it says is still
considered best practice.
Most of it, perhaps, but I'm not sure I agree with all of section 3
step (9). In particular, fallback to next MX rather than next address
broke; I had to split my dual-stack MX into two MX records, each
single-stacked but referring to the same actual machine, because of it.
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