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.