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In Outlook 2007/2010/2013/2016 to create a custom field: open the Customize View dialog. Right click on the header row above the message list and choose Custom or Customize View (Outlook 2007 and older) from the bottom of the menu. In Outlook 2010/2013, it's called View Settings.
Click Fields (Columns in Outlook 2010/2013) to open the Show Fields (Show Columns) dialog:
Click New Field button (It's called New Column in Outlook 2010/2013).
Create a custom field of the Yes/No type and add it to the view.
Click Other Settings. Enable in-cell editing so you can check (or uncheck) it to mark an item.
Return to Outlook. Click in the column you just added to mark a message.
Note: The actual checkbox may not be visible in the column until you click it.
In Outlook 2010 and Outlook 2013, you need to turn off Show as Conversation on View tab. When Show as conversation is checked, the field won't work when there is only 1 message in a conversation.
Create a notes field for messages
To create a field so you can add notes to the messages, you need to create a Text field and enable in-cell editing. Use the same steps as above, choosing the Text field type instead.