Remote server returned 550.5.7.705

Since 21/06/2024, I can't send emails anymore with my domainname 'adviesopmaat.eu'. It's works only with 'outlook.com'. The night before, I received a lot of spam + mails where send to unknown adresses with mine emailadress.
When I want to send an email, I receive back an a email from microsoft:
Undeliverable. Delivery has failed to these recipients or groups.
Your massage wasn't delivered because the recipient's email provider rejected it.
Diagnostic information for administrators: Remote server returned '550.5.7.705 Access denied, tenant has exceeded threshold.
I posted this issue to Microsoft support on 21/06/2024, but I got no answer back.

What can I do?

Thanks for helping!
Kind regards,
Wim