You email was filtered and I would not have seen the invoice at all had I not searched for it in my email. You can embed something that tells you whether or not the invoice was opened and if not send a reminder or something to rectify the situation. You can also try to send emails in more than one way so that filters will be avoided. Metrics on how long it takes to open emails can also be used for A/B testing different methods (of trying to avoid filters).