Trigger on send email with Template..

Trigger SendEmail on Invoice__c (after insert, after update)
{
List<Id> ContIds = new List<Id>();
 for(Invoice__c invoc :trigger.new)
  {
   if(invoc.Status__c=='Dispatch')
   {
   ContIds.add(invoc.Contact__c);
   }
 }
 if(ContIds.size() > 0){
   EmailTemplate e = [Select e.Body, e.Id, e.Name, e.Subject from EmailTemplate e where id='00XO0000000LwRu'];
   Messaging.MassEmailMessage mail = new Messaging.MassEmailMessage();
   mail.setTargetObjectIds(ContIds);
   mail.setTemplateId(e.Id);
   Messaging.sendEmail(new Messaging.MassEmailMessage[] { mail });
 }
}