You can control whether a new email will reopen a completed task.  By default, if a customer replies and a task is completed, Sortd will "Always" reopen the task.  However, if this doesn't work for you, you can set it to remain closed.

To change the setting, click on Sortd menu and hover over the board:

Then select the Board Settings icon as shown above.  Then choose one of the reopen completed items setting as shown on the right pop-up.