The bounced email should be 'bounced' back to the email address it was sent from.
i.e. if your board is sending from email@example.com
then it should come back to firstname.lastname@example.org
email address. I would suggest setting up a POP mail account for returned emails. Do people email you at the same address you send from? You need to have an email address inbox the same as the one being mass emailed from.
The bounced emails will have the email address it's being bounced from then it's a case of using your ACP to find the member by putting in the email address bounced in a search of find member and then either delete, deactivate or find another address to email them.