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Environment details? (see Admin panel -> Dashboard -> Information)
I do not recommend that you allow end Users to select the Priority of a ticket. You can set the priority based on the email priority if you want, but again that is allowing the end user to dictate the priority.
We set priority based on the Help Topic selected when the ticket is opened. (So things like Printer installations are low priority, and things like internet outages at a site are High Priority, etc.)
If you really want to allow the end User to select the priority be prepared to have the field not be a meaningful representation of the Priority, but you can do it.
Go to: Admin panel -> Manage -> Forms -> Ticket Details
Create a new field,
Type: Priority Level
Click Save Changes.