The odd thing, I setup a Gmail account to forward to company-support-12@discoursemail.com and that worked! But changing over to our actual Microsoft 365 support email managed to break things, so I assumed the error was with Microsoft.
OK, I have added this to the box,
`mycompany+support-12@discoursemail.com|support@mycompany.com
I will active email forwarding, send a test email and see…
Will report back.