Here’s my full payload. What I see is that your “TO” and “FROM” is the same based on your variable.
I guess you don’t want that, because TO and FROM should differ if you don’t want to write to yourself. 
{
"group_id": 1,
"priority_id": 3,
"state_id": 2,
"organization_id": 2,
"title": "API-Test that works anyway",
"owner_id": 3,
"customer_id": 6,
"note": null,
"first_response_at": null,
"first_response_escalation_at": "2019-01-31T10:50:28.000Z",
"first_response_in_min": null,
"first_response_diff_in_min": null,
"close_at": null,
"close_escalation_at": "2019-02-06T10:50:28.000Z",
"close_in_min": null,
"close_diff_in_min": null,
"update_escalation_at": "2019-02-01T11:20:28.000Z",
"update_in_min": null,
"update_diff_in_min": null,
"last_contact_at": "2019-01-14T15:50:28.000Z",
"last_contact_agent_at": null,
"last_contact_customer_at": "2019-01-14T15:50:28.000Z",
"last_owner_update_at": null,
"create_article_type_id": 5,
"create_article_sender_id": 2,
"article_count": 1,
"escalation_at": "2019-01-31T10:50:28.000Z",
"pending_time": null,
"type": "email",
"time_unit": null,
"preferences": {
"escalation_calculation": {
"first_response_at": null,
"last_update_at": "2019-01-14T15:50:28.000Z",
"close_at": null,
"sla_id": 1,
"sla_updated_at": "2019-01-15T08:30:55.830Z",
"calendar_id": 1,
"calendar_updated_at": "2019-01-17T10:53:45.906Z",
"escalation_disabled": false
}
},
"updated_by_id": 3,
"created_by_id": 3,
"created_at": "2019-01-14T15:50:28.000Z",
"updated_at": "2019-01-18T17:49:29.928Z",
"tags": "gets,ignored,anyway",
"article": {
"type_id": 1,
"sender_id": 2,
"from": "LeTest <alias@secret.tld>",
"to": "Users",
"cc": null,
"subject": null,
"reply_to": null,
"message_id": null,
"message_id_md5": null,
"in_reply_to": null,
"content_type": "text/html",
"references": null,
"internal": false,
"preferences": {},
"updated_by_id": 3,
"created_by_id": 3,
"origin_by_id": 5,
"created_at": "2019-01-14T15:51:50.000Z",
"updated_at": "2019-01-14T15:51:50.341Z",
"attachments": [],
"type": "email",
"sender": "Customer",
"created_by": "alias@secret.tld",
"updated_by": "alias@secret.tld",
"origin_by": "test2@test.com"
}
}