I am successfully able to create a PM just fine via the api as a user with a non-admin api key:
curl -i -sS -X POST "http://localhost:3000/posts.json" \
-H "Api-Key: 768dae6a45015f4220ad195ef9a8e8c4b48687a5b0bead26a6bbe2cf1a0f8b70" \
-H "Api-Username: steve" \
-F "title=I need to create a random title" \
-F "target_usernames=blake.erickson" \
-F "raw=This is the body of my pm that I am sending. I hope it is long enough" \
-F "archetype=private_message"
HTTP/1.1 200 OK
So far, all off the 422 responses that I was able to trigger have returned a json response in this format with an error message:
{"action":"create_post","errors":["Body is too similar to what you recently posted"]}
Some of the other error messages I was able to generate are:
One of the users you are sending this message to could not be found.
Title is too short (minimum is 2 characters)
and
Body is too short (minimum is 20 characters)","Body seems unclear, is it a complete sentence?
The “Body is too similar..” error will happen if a user sends the same pm to multiple different users, but an admin can successfully send the same pm body to multiple different users.
As long as we return an error message with the 422 I would say this endpoint is working as advertised.