I've got a user that send a load of messages that triggered the outbound spam policy and placed them on the restricted senders list. The 365 Defender investigations have completed (Partial remediation because I declined the password reset as it wasn't necessary).
User is removed from the Restricted users list and doesn't appear if you query from Exchange powershell. The outbound spam policy does not prevent sending for 24 hours.
They were receiving:
This message couldn't be delivered because the sending email address was not recognized as a valid sender. The most common reason for this error is that the email address is, or was, suspected of sending spam. Contact the organization's email admin for help and give them this error message.
Remote Server returned '550 5.1.8 Access denied, bad outbound sender AS(42004)'