The Libraesva Email Security Gateway Dynamic Recipient Verification is not working with Office 365.
To enable Recipient Verification in Office 365 you need to have Exchange Online Protection enabled on the server, as well as a Global Admin or an Exchange Company Administrator account.
The Directory Based Edge Blocking (DBEB) feature from Office 365 enables users to reject messages for nonexistent recipients.
To configure and Enable DBEB, use the following steps:
Ensure the domain is set to Internal Relay, by going to EAC > Mail Flow > Accepted Domains > Select your domain and click Edit > check if the domain type is set to Internal relay, if not change it to Internal relay and click Save.
Add your valid users to office 365 via Directory synchronization, remote Windows Powershell or directly from the Exchange Admin Center (EAC).
Now set your domain to Authoritative. Follow the same path as above, 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“.
Now you’re all set and you can use the Dynamic Verification feature of Libraesva Email Security Gateway
For more information check this article from Microsoft.