But how can you possibly deliver email via IPv6 if their MX host doesn't have IPv6 address at all?

    $ host hotmail.com
    hotmail.com has address 204.79.197.212
    hotmail.com mail is handled by 2 hotmail-com.olc.protection.outlook.com.
    $ host hotmail-com.olc.protection.outlook.com.
    hotmail-com.olc.protection.outlook.com has address 104.47.57.161
    hotmail-com.olc.protection.outlook.com has address 104.47.58.161

On the other side, if a host announces that they have an IPv6 address - do you think they do it mostly for spamers?

    $ host gmail.com | grep handled | head -n1
    gmail.com mail is handled by 5 gmail-smtp-in.l.google.com.
    $ host gmail-smtp-in.l.google.com.
    gmail-smtp-in.l.google.com has address 173.194.73.27
    gmail-smtp-in.l.google.com has IPv6 address 2a00:1450:4010:c1c::1a