But there are other status messages where the user account has to be updated. For example, when the mail can not be delivered because the mailserver can not be reached at all. There are other messages like "connection refused".
I found some error messages explained here: http://www.penpublishing.com/Support/EmailErrors/UnderstandingEmailErrors/
Then some users mailboxes are always full so the mail is not delivered. Some mailboxes are full and will never be empty. Do you filter out these messages, too? It is quite more difficult to check against whether the mailbox is full for a specific period. Or do you classify a mail as bounced the first time the mailbox is full? It is more uncommon this time to get this error. So these abandoned mailboxes might be from a different (internet) decade.
Which mailserver status messages do you classify as "bounced" in the way that you update your user accounts to "email bounced"?