3660713 - No valid recipients provided

• Observed error message "No valid recipients provided" in SLG1, /iwfnd/error_log (gateway error error log), etc.
• We can reproduce this issue by executing /n/IWNGW/BEP_DEMO, Provider ID = /IWNGW/DEMO_EXT, Recipients user has no valid mail address BTP user profile.
  /iwfnd/error_log results error like:
  
  The remote service has raised a Client Error '400': 'Bad Request'
  
  .ERROR_CONTEXT    
  ..ERROR_INFO    
  ..Exceptions    
  .../IWBEP/CX_CP_REMOTE    
  ....Text    The remote service has raised a Client Error '400': 'Bad Request'
  ....Attributes    
  .....CONTENT_TYPE    application/json
  .....HTTP_ERROR_BODY    {"error":{"code":"400","message":{"lang":"en","value":"No valid recipients provided"}}}
  .....HTTP_STATUS_CODE    400
  .....HTTP_STATUS_MESSAGE    Bad Request
  .....SAP_NOTE_ID    0000000000
  ....Source    
  .....Program    /IWBEP/CL_CP_SRV_PROXY_REMOTE=CP
  .....Include    /IWBEP/CL_CP_SRV_PROXY_REMOTE=CM006
  .....Line    55
  ..SERVICE_INFO    
  ...SERVICE_NAME    /IWNGW/CLOUD_NOTIFICATION_SRV
  ..SYSTEM_INFO    
  ...REQUEST_URI    /Notification.svcNotifications

SAP Fiori Notification

sap s/4hana, notifications, btp, mobile start app, error 422, unprocessable entity, notification delivery, email address, notification channel, pilot note, sap gwfnd, CLOUD_NOTIFICATION_SRV, Notification.svcNotifications, No valid recipients provided , KBA , OPU-GW-NOT , Notification , Problem