SocketLabs’ Hard Failure Score metric is based on the rate of hard failures that are recorded during the 15 day rolling period.
Hard failures are when the receiving mail server rejects an incoming message because the address is invalid, usually with an accompanying permanent "5xx" failure code. Failing to remove bad addresses from your lists can quickly lead to sender reputation issues. This is why when such errors are encountered, SocketLabs automatically adds the bad address to the Suppression List that we maintain on your behalf. We will not attempt to deliver messages to any address that is on the Suppression List.
A consistently high percentage of hard failures are an indication that the sending organization is not following email address acquisition and/or list management best practices. You cannot maintain a high rate of hard failures without consistently sending to new bad addresses. These bad addresses may not be new to you, however, as sending to old email lists can generate just as many bad failures.
For more information around SocketLabs' tolerance of hard failures and our suggested best practices, see our Acceptable Failure Rates article.