RFC 5321 is ambivalent of A vs. AAAA. When discussing the matter, 5321 says "A or AAAA RR". It later says about IPv6 that "The appropriate actions to be taken either will depend on local circumstances." When I re-wrote Gmail's DNS libraries about 6 years ago, I wrote it to start with AAAA and fall back to A. So if you have an MX and AAAA records for those names, you should notice all your inbound from Google comes via IPv6.

I've noticed that!

I believe Postfix also delivers to IPv6 if possible.

In my travels there's a significant portion of the internet which is not IPv6 capable; for example the World Community Grid failed for me when trying to use pure IPv6, and they send emails about things. I'd currently not risk using an IPv6 record for my MX, I suspect a high failure rate delivering email from every service on the planet will follow.

> set qu=mx > gmail.com. Server: 10.1.1.1 Address: 10.1.1.1#53

Non-authoritative answer: gmail.com mail exchanger = 20 alt2.gmail-smtp-in.l.google.com. gmail.com mail exchanger = 5 gmail-smtp-in.l.google.com. gmail.com mail exchanger = 40 alt4.gmail-smtp-in.l.google.com. gmail.com mail exchanger = 10 alt1.gmail-smtp-in.l.google.com. gmail.com mail exchanger = 30 alt3.gmail-smtp-in.l.google.com.

Authoritative answers can be found from: > set qu=aaaa > gmail-smtp-in.l.google.com. Server: 10.1.1.1 Address: 10.1.1.1#53

Non-authoritative answer: gmail-smtp-in.l.google.com has AAAA address 2607:f8b0:4001:c05::1a

Sure, but it has A records too.

I wake up and see the HackerNews downvote train has hit myself and you, when what you said is 100% correct; gmail-smtp-in.l.google.com is both IPv4 and IPv6.

The network I'm on right now (VPN to work) cannot service IPv6 and I cannot connect to the IPv6 IP for that MX host, but I can easily connect to the IPv4 endpoint.