When does a hard bounce set the email status to invalid?

Summary:

Lately we are seeing many emails go undelivered due to hard bounces despite trying to exclude them in segments. Reviewing the contacts listed, they all have a valid email status in Eloqua.

What are the rules for changing the status to invalid?

Which of the 5xx error codes trigger this status change?

Under what circumstances might a hard bounce occur that doesn't result in the contact's email being marked as invalid?

More details:

In a recent campaign, despite having this criterion set in the segment, we saw a lot of hard bounces:

In insight I ran the report Email Bounceback History with Messages