David Frank has created a free Mail.app plug-in known as MailPriority which enables the X-Priority header. Not only can you set the outgoing priority value, but with Mail.app's rules, you can color-code your email based on the value of the X-Priority header.
MailPriority relies on an unpublished Mail.app API, so it may stop working at any time. However, since it installs as a bundle in your local Library folder, the Mail.app program is not modified in any way, and you can remove MailPriority by simply deleting the program.
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