The role of SUBRL filtering in email security is to detect spam emails sent from third party devices by botnets. Rather than just filtering out emails from malicious IP addresses, SUBRL filters detect spam and malicious emails sent by botnets by analyzing the URLs embedded in the content of the email. If a URL redirects a recipient to a site known to be used by spammers or cybercriminals, the email is blocked, rejected, or quarantined depending on how the filter has been configured.