Microsoft 365: Delivery has failed to these recipients or groups

Error context

When using Microsoft 365 or Office 365 to send email to subscribers, I encountered the following error after a few days/weeks:

Delivery has failed to these recipients or groups:

***@***
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 email. Contact your email admin for assistance.

The program uses SMTP to send emails to thousands of users.

Root cause

Microsoft/Office 365 detects that I am sending similar emails to many users and marked the account as spam email account.

Resolutions

To fix this issue immediately and temporarily, go to https://security.microsoft.com/restrictedusers and then remove your legit user from the restricted users list.

If you want to whitelist your email account, follow these steps:

1. Visit thread policies: https://security.microsoft.com/threatpolicy.
2. Click Anti-spam policy.
   
   
3. Create a outbound policy by clicking '+ Create policy' button and then select Outbound:
   
   
4. Input a policy name:
   
   
5. Click Next button.
6. Input the user name:
   
   
7. Click Next button.
8. Input outbound protection settings:
   
   As the above screenshot shows, I've changed the restriction policy to alert only. 
9. Click Next button.
10. Review the settings and then click Create button to enable the policy.

References

Remove blocked users from the Restricted users portal in Microsoft 365