Omar, you can create a domain event to accomplish this.  In SmarterMail 16/17, go to domain settings > events.  Create a NEW event and give it a name/description.  Set the Event Category as User, the Event Type as User Added.  Leave the condition as the default.  Add a new "Send an email" action.  In the "To Address" field enter  #username#@#domain#  (make certain to include the # symbols around username and domain).  You can use the same formatting within the body of the message, if desired.