I think this header will be accepted by Outlook.. But lotus notes need the different format. Like
Importance: High
But the Mime:lite module omits it i believe. the other headers x-priority and other shown successfully.
But it not shown visually.

In reply to Re^8: High importance mail in lotus notes using MIME::Lite by SriniK
in thread High importance mail in lotus notes using MIME::Lite by SriniK

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