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For classic Outlook, in the message list, select one or more messages, then select Home, and in the Tags group, selectĀ
 Ā Categorize.(If you haveĀ aĀ message open, select Message > Categorize.)
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Select a color category.Ā The first time you select a new category, you can:
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Add a more descriptive name.
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Add a keyboard shortcut key. This lets you quickly assign categories on selected messages by pressing the shortcut key.
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Customize the color.
 
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After you assign a category to a message, in the message list, the tag is visible in the Categories column. And when you open the individual message, the tag is also visible in the message header.
To categorize meetings or appointments, seeĀ Assign categories to meetings and appointments.
Tip:Ā For classic Outlook, select  
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For new Outlook, in the message list, use Ctrl+click to chooseĀ one or moreĀ messages. Select Home and then chooseĀ
 Ā Categorize.If you don't seeĀ
 Ā Categorize, at the right end of the toolbar, selectĀ Ā More options. - 
                      
Select a color category from the menu.
Or, if you want to create a category of your own, select New category. You can choose aĀ name and color for your custom category.
 
After you assign a category to a message, in the message list, the tag is visible in the Subject column. And when you open the individual message, the tag is also visible in the message header.
To categorize meetings or appointments, seeĀ Assign categories to meetings and appointments.