SITE_EMAIL_SEND SITE Endpoint
  /rpc/site/?method=SITE_EMAIL_SEND  
  This method is identical to MESSAGING_EMAIL_SEND, with the restriction that you can only access objects created in the current session etc  
     
  You will also need to set up a messaging account, using SETUP_MESSAGING_ACCOUNT_MANAGE, that matches the &fromemail and has &siteemail=Y.