The necessary modification needs to be made in SD.Email.Monitor
Note that the only valid priorities will be the build it defaults (Low, Minor, Normal 'the default', High, Urgent, Emergency).

1. Open SD.Email.Monitor in Workflow Manager.
2. Open the ProcessMessage linked model.
3. Edit the 'Create Incident' component.
4. Click the Ellipses for Mapping Definition. There are currently only 3 mappings.
5. Scroll down to Priority on the right hand side.
6. Right click on Priority and choose 'Create ConstantMappingAssignment.
7. Double click the new constant link and set the value to 'Low' or one of the other built in priorities (High and Emergency would not be good choices) 
8. Click OK 3 times to get out of the editor.
9. Save & Publish the updated project.
10. Run an 'IISreset' from a command prompt and restart the 'Symantec Workflow Server' service.

From now on Incidents created from emails to the ServiceDesk email box will will initially have the priority set to 'Low'.