What does updated via BounceProcessor mean?

In the Audit Log that found on individual contact records you can view certain recent actions on the record in the system. One such action that is possible to be seen in the Audit Log would the the processing of an unsubscribe. In the "Updated Via" column, if the value shows "BounceProcessor", this means that the email recipient took some action to tell their system that they didn't want the email. See this example record:

When this occurs the recipient's system reads the email header. Every email header contains a list-unsubscribe. The list-unsubscribe contains a unique address that will automatically unsubscribe the recipient from the account from which the email was sent.