[ipv6-wg] Real-world IPv6 SMTP experience

* Gert Doering:

> So if I understand this correctly, having a v4-only backup MX should
> solve all problems commonly experienced?

As far as I know, the primary MX needs an A record (to satisfy
Sendmail), and there needs to be a MX which has an A record only (and
no AAAA or A6 record; this is done to satisfy Exim installations).  My
initial question to this list was whether if you can do this with just
a single IPv4 address, and still prefer IPv6 over IPv4 (assuming that
IPv6-capable hosts prefer IPv6 hosts over IPv4 hosts if they have the
same priority).