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 -> scroll to the Prospect Category field -> change the category -> and click Update.