23. The invention of claim 19 , further comprising: extracting a domain name portion of the third network resource identifier; retrieving one or more NS records for the domain name portion from a domain name system (DNS) server; retrieving network address records for each mail exchange that is identified in the one or more NS records; determining an average reputation score for the network address records; adding the third network resource identifier to the blocklist when the average reputation score is less than a specified threshold.
23. The invention of claim 19 , further comprising: extracting a domain name portion of the third network resource identifier; retrieving one or more NS records for the domain name portion from a domain name system (DNS) server; retrieving network address records for each mail exchange that is identified in the one or more NS records; determining an average reputation score for the network address records; adding the third network resource identifier to the blocklist when the average reputation score is less than a specified threshold. 24. The invention of claim 23 , further comprising 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.