Hi greengiant,
Based on the NDR via your private message, I have checked your domain DNS settings and found the MX record was incorrect. I suggest you follow the steps below to see the correct MX record and add it to your domain.
1. Sign in to Microsoft Online Portal and click “Domains” under “Management” in the left panel.
2. Select your current domain name and click “Troubleshoot”.
3. Select “I changed the settings at my provider more than 72 hours ago” if you did.
4. Click "Next".
Thanks,
Allen Zhang