Can I mark an Email as "High Importance" for Outlook using System.Net.Mail?

Question

Part of the application I'm working on for my client involves sending emails for events. Sometimes these are highly important. My client, and most of my client's clients, use Outlook, which has the ability to mark a mail message as High Importance.

Now, I know it is callous to assume that all end users will be using the same interface, sp I am not. But considering you can send email from Outlook as High Importance even if the target is not necessarily reading through Outlook, that means that there is basically some data stored, somehow, that lets Outlook know if a particular message was assigned as High Importance. That's my interpretation, at least.

The application currently uses System.Net.Mail to send out emails, using System.Net.Mail.MailMessages for writing them and System.Net.Mail.SmtpClient to send them. Is it possible to set this "High Importance" setting with System.Net.Mail's abilities? If not, is there any assembly available which can configure this setting?

Answer 1

Set the Priority property of the mail message. Its values are Normal, Low or High.

As @StefanSteiger notes, Priority is only guaranteed to work for Outlook. In the intervening 8 years since this question/answer were posted, the industry has settled on the Importance header as the preferred way to do this.

The source for MailMessage makes it clear that setting the Priority actually sets three things: the XPriority header, the Priority header, and the Importance header. So using the Priority property will behave as expected in any mail client, and will set the appropriate headers.