To change the priority of one item, open an item view, click the Send Options tab, then click General.

or

To change the priority of all items you send, click Tools > Options, then double-click Send.

Select High Priority, Standard Priority, or Low Priority.

The small icon next to an item in the Mailbox is red when the priority is high, white when the priority is standard, and gray when the priority is low.

Click OK.