This gauge counts the number of DNS blacklist lookups for the corresponding index that failed to complete, as opposed to succeeding with either a match or a clean result. It alerts an administrator to problems reaching the external DNSBL service itself, such as DNS resolution failures or the blacklist provider being unreachable. For example, a rising fail-check count while checked and matched counts stay flat would point to a connectivity or DNS issue between the firewall and that blacklist provider rather than a change in spam volume.