In my opinion it's actually something nice. They cannot stop accepting mail from all 'improperly' configured servers. So they decided to impose their standards by making you comply with them once you turn on IPv6. So they are using the IPv6 transition to make sure everybody sets up SPF records and such. In the long run this might help fight spam. However their total lack of feedback on why mail gets blocked is frustrating. There's probably some reason for rejecting your legitimate mail, but the unwillingness to tell you what that reason is totally sucks.