Once a conversation is closed, it is removed from the queue. However, you can still retrieve it using Quick Search or Advanced Search.
Closed conversation can be re-opened, which will mark it as open and make it visible again in the queue it was originally stored in. If it wasn’t stored in any specific queue, it will be marked as open and appear in the organization’s default queue—meaning it can only be found via Quick Search or Advanced Search.
Please note: if the conversation is closed and it was generated by an outbound campaign schedule, when a new inbound reply is received, it will be added to the conversation that will be automatically re-open and be visible in the latest queue where it was stored (if not stored in a queue, the conversation will re-open but stays on the organization default queue.