When somebody sends me an e-mail that is marked as "High priority" it does not show up any differently than other e-mails. I am expecting to see a red flag or other indicator to show that the e-mail has been marked by the sender (which could, for instance, be my boss) as more important or urgent and in need of immediate attention. Other e-mail clients I have used in the past did this automatically.
I have poked around a bit in the preferences and in the menus but I haven't found anything to enable such a feature. How do I enable Apple's default mail app to show me the information in the overview of my messages? (My mail window comes up with mailboxes/folders on the left, followed by a list of e-mail summaries (sender, date, subject, and the first few words of the e-mail) to the right of that, and on the right is the actual contents of whatever message is selected. I don't see any indication of priority anywhere.
Technical note: the sender is setting the following headers to indicate priority:
X-Priority: 1 (Highest)
X-MSMail-Priority: High
Importance: High
Best Answer
My recommendation is to use Apple Mail's rules to customize how you want these displayed—you can make them stand out even more than a simple !! in the message list by setting up a Rule in Preferences:
In this case, specify that you want to apply the rule to messages with high priority. Then choose the formatting that you want. I chose to list messages with a red background. When new messages arrive with a high priority, they got a red background in the message list.