Bluehost/Hostmonster IPv6 SMTP black hole?

Can someone from Bluehost/Hostmonster contact me off list. One of your
customers has complained to me that they cannot send mail to us.
We've done some testing, and it seems it works to mailservers with
only A records, but breaks with mailservers with both A and AAAA. (As
in, we never even see a delivery attempt.)