How can I use the Mail System module to send one e-mail differently than others?
These are the keys uses by the core user module:


'register_admin_created': Welcome message for user created by the admin.
'register_no_approval_required': Welcome message when user self-registers. 
'register_pending_approval': Welcome message, user pending admin
approval.
'password_reset': Password recovery request.  
'status_activated': Account activated. 'status_blocked': Account blocked. 
'cancel_confirm': Account cancellation request.
'status_canceled': Account canceled.