15. The invention of claim 11 , further comprising: means for extracting a domain name portion of the third network resource identifier; means for retrieving one or more NS records for the domain name portion from a domain name system (DNS) server; means for retrieving network address records for each mail exchange that is identified in the one or more NS records; means for determining an average reputation score for the network address records; means for adding the third network resource identifier to the blocklist when the average reputation score is less than a specified threshold.
15. The invention of claim 11 , further comprising: means for extracting a domain name portion of the third network resource identifier; means for retrieving one or more NS records for the domain name portion from a domain name system (DNS) server; means for retrieving network address records for each mail exchange that is identified in the one or more NS records; means for determining an average reputation score for the network address records; means for adding the third network resource identifier to the blocklist when the average reputation score is less than a specified threshold. 16. The invention of claim 15 , further comprising means for determining a blocklist status for the network address records, and adding the third network resource identifier to the blocklist when the blocklist status indicates that network addresses in the network address records are blocked.