https://userapps.support.sap.com/sap/support/knowledge/en/2013580
2013580 - Email notification for MLFH and MLGR is not sent- SAP ERP & SAP S/4HANA | SAP Knowledge...
Email notification for MLFH (GR Missing Parts MSg) and MLGR (GR Message) is not sent. In SBWP , you are able to receive the notification via system messages .