For Smartermail 8x - 15x:

Log into your SM admin interface.
Click Settings on the left.
Expand Defaults and choose User Defaults.
Click the Compose tab.
Put a check in the box that says "Enable Sent Items Folder"
Click Save.
Click User Propagation on the left.
Click the Compose tab.
Put a check in the "Enable Sent Items Folder" box.
Click the Propagate Now button at the top of the screen.
 
This will enable the Sent items folder for everyone on your server if it is not already enabled and any new users you add will have the Sent items folder enabled by default.