Enable Recipient Filtering on Exchange Online (Microsoft 365)

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:

1. Login to the Exchange Admin Center
2. On the left-hand sidebar, navigate to Mail flow -> Accepted Domains
   
   
3. Click on your default domain
4. Make sure the domain is set to Authoritative
   
   
   
   
5. 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:

https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/use-directory-based-edge-blocking