When you want people to know your message needs urgent attention, set the message as high importance. If the message is just an FYI, or if you’re sending mail to colleagues about a non-work related topic, you can set the message as low importance.
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Click New Email to compose a new message.
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In the Tags group on the Ribbon, select High Importance
or Low Importance 
.
In most email programs, the recipients see a special indicator in the message list or headers for your message.
From any message you're currently composing, you can tell when an importance has been set by looking at the Tags group on the Ribbon. If High Importance or Low Importance are highlighted, they are set.
Change the importance of an email you've received
If you want to change the importance of an email you've received, you can edit the email message.
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Double-click to open a received email message. You cannot change importance while viewing a message in the Reading Pane.
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In the Tags group on the Ribbon, click the More arrow to open the Message Options dialog.
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Use the Importance drop-down to change the importance level, then click Close.
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Close the message, and when Outlook asks you if you want to save changes, say Yes.
