It wasn't clear to me as even at that point it was an "H1 Mediator" who responded.
Also the bit about SPF, DKIM and DMARC seems to show a misunderstanding of the issue: these are typically excluded because large companies aren't able to do full enforcement on their email domains due to legacy. It's a common bug report.
In this case, the problem was that Zendesk wasn't validating emails from external systems.