I always see the below line in my postfix logs,
status=sent (delivered via filter service)
Does that mean message reached mailbox ?
There are other logs like
Accepted for delivery
Delivered to mailbox
No, it means it was delivered to the "filter" service. What that filter service does is not indicated in this log entry. If you search the message ID, you should find more about what happened next.
There are good books on postfix such as http://www.postfix-book.com/ - I recommend it to you. This may give you a head start on understanding the logs (although I think understanding the logs requires understanding the system) http://en.redinskala.com/postfix-maillog-interpretation/