When a prospect replies to your emails, sometimes they are automatically categorized into Do not contact, Auto-reply, Out-of-office, Forwarded or Delivery failed based on the type of reply received. 

You may manually change the prospect's category by:

First way: In the Inbox, click on an email conversation, and then change the "Category" on top. This will change the prospect's category. 

Second way: You can also go to the Prospects page -> Select a prospect by clicking on the email -> Click on Prospect Category field -> change the category

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