If a user continuously sends emails from Office 365 that are classified as spam, they will be restricted from sending any more messages outbound. The user will be listed in the service as a bad outbound sender and will receive a Non-Delivery Report (NDR) that states:
- Your message couldn’t be delivered because you weren’t recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it’s no longer allowed to send messages outside of your organization. Contact your email admin for assistance. Remote Server returned ‘550 5.1.8 Access denied, bad outbound sender’
Login to https://admin.microsoft.com
Exchange Admin Center, and then click Protection> Action center to check if your account is blocked.
Note: This needs to be completed by a global admin in your Office 365 organization. Please let your admins do this if you aren’t one.