Within Teamwork Desk, you can set a default status that should be pre-selected in the status field when you are replying to tickets.
To set a default status, click your profile and select Settings from the dropdown menu.
In the settings area, select the Inboxes tab and click the inbox you would like to set the default status for.
While viewing the individual inbox, select Edit Inbox in the left pane.
Hover over the Default Status option and click the edit pencil icon. You can then select the default for the inbox from the dropdown.
When you are creating a new ticket or reply to an existing one, the default status will be preselected for you. On individual tickets, you can click the status dropdown to choose an alternative status than the default.
An inbox can be set to prevent replies to closed tickets. When this setting is turned on, closed tickets become read-only and cannot receive any future replies, notes, or forwards.
Therefore, if Closed is set as the status on a ticket when an agent sends their reply, that ticket will be flagged as read-only and any attempted replies to the thread from the customer will be created as new tickets.
For more information, see: Ticket Statuses