I am a Yammer network admin, our data retension is Soft Delete, from this link ,
Retained data stored in Yammer can be permanently deleted by using the Yammer Developer API. To do this you export the data to identify the data that needs to be permanently deleted, and then write a custom PowerShell script to loop through the specific items to delete.
I can use API to export the soft deleted messages, but when I want to permanently delete it, the response is 400, Bad Request;
The api I used is https://www.yammer.com/api/v1/messages/:message_id, _method=DELETE
Could you give me some suggestion?