Define partner function DP as mail partner

What causes this issue?

The system tried to create the partner function DP (MRP controller for
missing parts) as a mail partner. However, the partner function DP
already exists and is not defined as mail partner (partner type MP).

System Response

The system can only create missing parts messages for the mail partner
function DP.

How to fix this error?

Check for which output types the partner function DP is
used.
Define a new partner function which is to replace the current partner
function DP.
Replace the partner function DP in the respective output types by the
new partner function.
Assign the partner type MP (mail partner) to the partner function DP.
<DS:TRAN:VOPA>Proceed</>
Restart the program.

Error message extract from SAP system. Copyright SAP SE.