How to get an project's emails/threads via api

I am trying to get a project’s emails or email threads via API.

I read through the documentation, and although not clearly described, I see I can request an item’s sub-objects by appending the sub-object to the master object.

Both combinations I tried don’t seem to work:
projects/711/mailThreads
projects/711/mailMessages

I also tried all typical parameters for the mailbox endpoints, but again none of this work.
I assumed that a similar api call must happening when I open a project in pipedrive where the project’s email threads are retrieved and from the email threads the email messages and so on.

/mailbox/mailThreads returns all threads and I can see the thread contains the “project_id”: 7408, but using that as a filter like &project_id=7408 also didnt work.

So how can I get a project’s threads and messages via API?