This should be the domain of your forum, not your email. Can you confirm that your forum is not at inbound.[mydomain].org? E.g. if meta were using this method, the url would look like this:
DISCOURSE_MAIL_ENDPOINT: 'https://meta.discourse.org/admin/email/handle_mail'
Also.. this has nothing to do with mailgun, except to make sure to ignore mailgun’s instruction to add MX records for the domain. As per:
Note: outbound email providers like mailgun may ask you to add MX records pointing to their servers. You want to remove these so the MX records for your forum only point to your forum’s domain name. SPF and DKIM records must still point to your outbound email provider servers so you can send email.