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Normally, to turn on the Nudge option, a user would go to the Settings gear, see all settings, and scroll down to check the boxes to suggest emails to reply to or to follow up on:
Occasionally, a user will look in their settings and will not be able to turn Nudges on because the option doesn't appear. The solution in this case is to turn Conversation View on - if Conversation View is off, a user will not even see the options for Nudges.