Exchange Online doesn't support recipient filtering but you can still configure Exchange Online to reject invalid recipients at the SMTP level. In Exchange Online, this feature is called Directory-Based Edge Blocking (DBEB). Here are the steps to enable this feature:
- Login to the Exchange Admin Center
- On the left-hand sidebar, navigate to Mail flow -> Accepted Domains
- Click on your default domain
- Make sure the domain is set to Authoritative
- Click Save. This will enable Directory-Based Edge Blocking and prevent Exchange Online from accepting mail for invalid recipients.
You can find more information about Directory-Based Edge Blocking here: