Enable Recipient Filtering, for Recipient Verification in Exchange Online (Office 365)
To enable recipient filtering in Exchange Online, you need to have Exchange Online Protection enabled on the server, as well as a Global Admin or an Exchange Company Administrator account:
- Ensure the domain is set to Internal Relay, by going to Exchange Admin Center > Mail Flow > Accepted Domains > Select your domain and click Edit.
- Ensure the domain type is set to Internal relayand click Save.
- Ensure that all desired mailboxes are configured in Exchange Online as the next step will block mail delivery to any addresses that are not listed.
- Set your domain to Authoritative by going to Mail Flow > Accepted Domains, select your domain and set it to Authoritative. After you click Save, please confirm that you wish to enable Directory Based Edge Blocking.
For on-premise Exchange, see: How to enable recipient filtering on an Exchange 2013 and 2016 server