I’m having a little trouble understanding your use of domain names. On my site the DISCOURSE_MAIL_ENDPOINT and MX record for inbound email is the same domain, pointing to the Discourse server, as explained in the OP in Straightforward direct-delivery incoming mail. You seem to be trying to use different domains.
Generally you want to use e.g. forum.mydomain.org type subdomain for your Discourse, to separate your forum from your main website and email on mydomain.org.