The Clean Up command is a quick and effortless way to reduce redundant emails. A “Conversation” in Outlook is essentially an email thread starting from the first email sent out down to the last reply.
When you hit the “Clean Up” button, messages that are already contained within one of the replies are deleted, thus reducing redundant messages. This feature can reduce your inbox messages by 35% quickly and efficiently without deleting anything you might need. Microsoft recommends keeping under 5,000 messages.