iDashboard will automatically unsubscribe contacts form email alerts if three consecutive email alerts bounce in a row (usually the period of three weeks). When this happens a note is also written to the contacts individual profile stating they were unsubscribed automatically by the system due to an invalid email address.
Emails often bounce due to invalid or deleted email addresses or full inboxes - it is safe to say that if an inbox is unavailable or full for three weeks in a row, the owner is most likely no longer using it.
If an email bounces once due to a network error or a temporary inbox error, the 'three bounce rule' described above will prevent the contact from been unsubscribed automatically.
Automatic bounce unsubscribing is a system-wide feature and can not be altered or turned off.