By default, Keeping is set to automatically close any ticket when you reply. When your customer replies, the ticket will be automatically re-opened. If you'd prefer for a ticket to stay open when you reply, you can adjust this in the settings for an individual mailbox. Navigate to the mailboxes page in Keeping and choose the mailbox you wish to edit by selecting the pencil icon.
Scroll down, and tick the box next to "Automatically close ticket when agent replies" to disable automatic closing. Don't forget to click Save when you are done.