Again and again I hear '...but our ESP says over 95% of emails were successfully delivered!' when asking about deliverability situation. To be honest - this makes me sad. No, your ESP has no idea what has happened with an email since it was accepted by a mailbox provider. And it's really looking like scam - ESPs saying 'delivered' while it's really 'accepted'.
And there's a huge difference between delivered and accepted. Since an email has been accepted by MBP it might end up in Inbox or Spam. Or it might be discarded so a receiver will never see it. At the same time there might be a time lag between acceptance and show up in an inbox.