Attempts to send a Zivver message result in a non-delivery report (NDR), a bounce message. The bounce message is as follows:

5.3.0 Unable to specify specific reason due to missing diagnostic code[delivered; 5.3.0]

As the bounce states, the message is successfully delivered. But the sender gets a bounce.

For the error Bounce 5.0.0, refer to Bounce 5.0.0 Unable … diagnostic code.

Cause

The receiving mail server returns an incorrect Delivery Status Notification (DSN). While few providers request Success DSNs, Zivver uses them to confirm successful message delivery.

The message shows that the message was delivered, but a status 5.3.0 is returned. That means that authentication is required.

Diagnose
Send a test message to the affected domain in a setup that requests a Success DSN.

Solution
Speak to the receiving party about the issue so that they can resolve it on their end.