Question about Inbox API "User does not have permission to access inbox"

Hi, I've noticed that whenever I try the inbox API eg. https://api.servicem8.com/api_1.0/inboxmessage.json
It is giving the response "User does not have permission to access inbox"

About our account:
-API key type: full access
-Current Plan: Growing
-Email inbox addon: installed and active
-Was able to send and receive emails through the UI?: Yes

However, everytime we try the inbox API it is giving the error.

I wonder if we have missed enabling something?

Thanks,
Ralp