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I guess nowadays that should still be useful to give both an IPv4 and IPv6 address for the mail server?

Yep, just have A and AAAA records for the hostname the MX points to

Why? You can just make mail.example.com resolve to two IPs: an IPv4 and IPv6 address. Nothing wrong with that.

Isn't that what Natsu said?

No. If you read the top-level comment, it sounds like the response directly indicates that these days it would be useful to use the defunct multiple classes (IN vs CH) functionality to instead reference different IP types (v4 or v6) in today's Internet, hence "nowadays". That isn't needed because we can already mix v4 and v6 records within the IN class itself through A/AAAA records.

If I misunderstood the comment, then my bad.

I see, I haven't had to dig too deep into DNS so it wasn't obvious to me that those were separated like that. I guess, then, that it's only useful to allow us to someday upgrade to some post-TCP/IP world if there's ever a reason for that.

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