You need to think through all the interactions before enabling IPv6 on a mail server.
Ket Crispin posted this to the IETF list:
I was presenting what I thought was an interesting example
The mailserver in question uses a default redhat enterprise
Now I know different. Just enabling ipv6 on an otherwise
As I mentioned, my servers actually do reject mail if they
Paul Warren adds:
Google mail servers reject mail with no IPv6 reverse DNS. This means that a) if you adopt this measure then you're in good company, and b) if you enable IPv6 on your mail server, then it's very important that you have working IPv6 reverse DNS.
When enabling IPv6 on a mail server, you need to consider all the places where you might have IP-based access restrictions in place, as even if you don't modify add any AAAA records to point at your server, it will start using IPv6 for outgoing connections. One place to consider is SPF records for any domains for which your server sends mail.