CiviMail report showing bulk emails have sent when they haven't
Glad you got it working back.
smtp protocol is made to be very resilient, but it means as well it's really hard to know for sure when/what is going wrong.
I'm not sure I understood the details of your configuration, but a somewhat common problem is when you have a local satellite mail server (that will always tell civi the mails are sent as it accepts them) and that isn't able to send it forward for whatever reason... and report that issue in a mailbox that isn't properly read by civi (for another problem).
So from civi point of view, it's all working fine, and in that specific case, it's very difficult to identify there is a problem (or two, as one hides the other), unless you add human brains that is able to identify that it shouldn't have zero bounces and investigate further.
Beside recommending to validate the bounce process and checks that it works on a regular basis, not quite sure how civi can do better on its own.