Sending mail with Outlook high priority

Is there any PHP functionality for sending mail and attaching a high priority to the mail item ? In Outlook ( and Outlook Express ) there is the notion of a high priority mail item, but I do not know if this corresponds to anything in an RFC for mail. Nor do I know how to mimic this in PHP. If anybody does or could point me to the right functionality, it would be appreciated. Currently I am using the mail() function that is standard in PHP.