[ipv6-wg] Experiences with IPv6-only MX as primary?

Hello,

I set up a mail server zone to have an IPv6-only primary and a
dual-protocol secondary, like this:

; SMTP chooses lower-priority first, so smtp-primary is preferred
example.com. 300 MX 10  smtp-primary.example.com.
example.com. 300 MX 100 smtp-secondary.example.com.

; smtp-primary is IPv6-only
smtp-primary.example.com. 300 AAAA 2001:DB8::1

; smtp-secondary has both IPv6 and IPv4 addresses
smtp-secondary.example.com. 300 AAAA 2001:DB8::2
smtp-secondary.example.com. 300 A 192.0.2.11

Mail delivery via IPv6 works fine (as you would expect), and mail
delivery from some IPv4-only servers also works fine (as I think the
standards dictate).

My concern is that some mail servers may choke on this setup. For
example, http://mailtester.com complains "Network is unreachable".

Of course, I also worry about all the spammers who may not be able to
deliver because of incomplete mail implementations on the trojans and
other viruses infecting their zombie hosts. 😉

Does anyone have any experience with a similar setup? Have there been
problems?

Cheers,

--
Shane