Restricted Users

Office 365 may block users from sending e-mails if they are suspected of being compromised. To see the list of users and remove them from the list:

https://security.microsoft.com/antispam

To set who gets a notification when a user is blocked:

https://security.microsoft.com/antispam

Anti-spam outbound policy (Default)

Edit Protection settings

Notify these users and groups if a sender is blocked due to sending outbound spam