Background:
When sending IDOCs from SAP ECC via RFC port, they first will get status 03 when they are dispatched to the port. Nevertheless delivery could still fail (they could hang in SM58). If they are finally delivered to the target, the status will switch to 12 (by use of the transaction BD75 or the program "RBDMOIND") as explained in this blog by Vadim Klimov.
Questions:
When using an "XML-HTTP" port (instead of RFC) in WE21 and sending out IDOCs to the target via a HTTP destination, the IDOCs stay in status 03 (even if they were successfully sent out and reached the partner).