I think you misunderstood the issue. (and I did mention in the opening post that our DNS settings have been confirmed seveal times)
These are existing contacts in our CRM who regularly receive and respond to direct emails from"ken@ourdomain".
A significant percentage of these contacts did not receive the marketing email, sent from “ken@ourdomain”. No bounces, none in junk folders, none in spam folders.
Clearly, these emails are being quarantined by a firewall or enterprise spam filter… despite direct emails from “ken@ourdomain” being allowed through to the recipient.