If you login to Process Manager
Admin | Data | Application Properties
Is there a 'ServiceDeskSettings' listed?
If you click on this, there is a section for Mail Settings, check these are filled in correctly.
Back in Workflow Manager open 'SD.Email.Monitor'
Click on the Project in the Project treeview
Click on the 'Application Properties' tab.
Use 'Application Properties' should be ticked.
'ServiceDeskSettings' should also be ticked.
If these aren't, check them then you can click Refresh too.