Hi lost_but_happy,
I have analyzed the NDR and noticed that the MX record of your domain hasn’t been set up correctly.
You can add the related DNS records manually with the steps below:
1. Login to Office 365 portal (https://portal.microsoftonline.com/) with the administrator account.
2. Click “Domains” which is on the left side under “Management”.
3. Click the domain which had been verified in the list.
4. Click “DNS manager” which is beside “Domain Properties”, then you can find the related DNS records as the screenshot shows below:
5. Update the MX record, then add the related CNAME record and TXT record that the screenshot shows above.
Thanks,
Wilfred Ying