Problem when using the FndStreamMessages EntitySet of the StreamSubscriptions projection

Hello everyone,

I would like to send a stream message using the "StreamSubscriptions" projection and the "FndStreamMessages" EntitySet.
By doing some tests with Postman, I can send a stream message perfectly but...
It only works when I send a message with 
- Sender : IFSAPP
- Recipient: IFSAPP
or 
- Sender: IFS user
- Recipient: IFSAPP

On the other hand, if I try to send a flow message with :
- Sender: IFSAPP (or IFS user).
- Recipient: IFS user.

I get this error when running my web service:

For information, here is what it has in the body when the error appears (of course the user exists):

{
"FromUser": "TBO",
"ToUser": "TBO",
"Message": "Test",
"Header": "Test",
"Visible": true,
"StreamType": "General",
"Read": false,
"FollowUpCreated": null,
"Url": null,
"Notes": null,
"LuName": null,
"Reference": null,
"ReferencedObjkey": null,
"WebUrl": null,
"ItemId": null
}

Do you have any idea?
Thanks in advance,