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What is a AAAA record?
The AAAA record is the IPv6 counterpart of the A record: it points a hostname at an IPv6 address.
A AAAA record maps a name to a 128-bit IPv6 address. Apart from the address family it behaves exactly like an A record.
A host reachable over both protocols carries an A and a AAAA record for the same name. Clients then choose, usually preferring IPv6 when it works.
This matters for mail: a sending server that picks IPv6 needs valid reverse DNS and passing SPF or DKIM over that address, or large receivers will reject the message. Publishing AAAA without checking the IPv6 side of authentication is a common cause of sudden delivery problems.