You have configured the E-mail Notification but still no outbound messages appear on SOST and no e-mail is received by the user for some specific tasks that reach user's inbox.
Check if the task is specified in the following IMG activity:
SPRO > Governance, Risk and Compliance > General Settings > Workflow > Workflow E-mail Notifications > Maintain Workflow Notifications (Business Scenario > Filter Basic Data > Filter Settings).
If the task is not specified add it to the filter so that when such an item comes in the work inbox, e-mail notification is triggered.
SAP Note: 1784868 GRC 10.0 Email notification is not sent for Incident workflow.