This error message highlights that the email address is suspected of sending spam. It is then blocked from send messages outside of the organisation.

This normally happens when a user needs to send emails in bulk, which gets their account locked.

The account will get restricted. The restriction will need to be removed on the Microsoft 365 Defender portal. https://security.microsoft.com/restrictedusers

Select the user and ‘unblock’ them.

It will take up to an hour for this change to follow through.

Reference:

• https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/removing-user-from-restricted-users-portal-after-spam?view=o365-worldwide