When you send an email, the email is not delivered, and you receive a non-delivery report with this error message:
Microsoft 365 has added the user to the blocked sender's list because the user has been sending emails recognised as spam or exceeding the limits of the Microsoft 365 plan, for example sending emails to a large number of recipients.
To resolve this issue see Remove blocked users from the Restricted users portal in Microsoft 365.
Before removing a blocked user, it is recommended you verify their account or computer has not been compromised and is not being used to send unsolicited emails.
For more information see How to determine whether your Office 365 account has been compromised and How to fix a compromised (hacked) Microsoft Office 365 account.
You can configure Microsoft 365 to send notifications when an outbound spam email is detected, or if a sender is blocked due to sending outbound spam. For more information see Configure outbound spam filtering in EOP.