Contribution pages - When email addresses are defined for the Receipt, CC Receipt To / BCC Receipt To fields and emails to those recipients bounce then that bounce is counted against the Contribution Contact and not the actual bounced recipient
If you set the Receipt email addresses on a Contribution page and ANY of those email addresses bounce then that bounce is recorded against the CiviCRM Contact that the receipt was sent too. So if you have 3 or more CC/BCC email addresses defined and they ALL bounce. Then that CiviCRM Contact's email is immediately put ON HOLD.
This is because the receipt email has the bounce return path of the CiviCRM contact and a single email is sent with the CC/BCC defined.
A possible workaround would be to send individual emails for each of the email addresses defined in the CC Receipt To / BCC Receipt To fields.
A symptom of this problem occurring is that new members, event participants and donors have their email addresses immediately marked as on hold after contributing or registering for an event.