Bounced email: [HM08] Message rejected due to local policy.

Hi - I can't get hold of anyone at the postmaster account for Apple whom I emailed earlier in the week. I have a mail server using DKIM and SPF, and sending mail successfully to loads of other domains / mail servers. Apart from the disreputable uceprotectl3 list which has blacklisted the entire subnets for the hosting company I am using (Digital Ocean), I am not on any blacklists.

From the server, icloud will accept emails from one domain but not another (I setup an icloud email address to test). The smtp error message refers to local policies, but links to a generic spam advice posting. I am not sending unsolicited email, and in fact the emails are human replies to ones received from icloud customers. Unfortunately the smtp error message in the logs doesn't say anything useful like what actual policy is causing the message being rejected.

Does anyone know what we can do to get Apple to deal with or help us diagnose mail delivery issues like these?