The GSuite Gmail Malicious SMTP Response rule implements a detection mechanism to identify and alert on potentially harmful email transactions processed by Gmail's SMTP server. This rule focuses on monitoring inbound SMTP connections and flagging emails that Gmail rejects for various security reasons, such as the detection of malware, identification of spam or phishing links, low sender reputation, relevant Real-time Blackhole List (RBL) listings, or activities indicative of denial of service (DoS) attempts. By examining the reasons provided by Gmail for blocking or rejecting emails, organizations can respond promptly and mitigate risks associated with malicious email threats. The rule aggregates log events from GSuite Activity Event logs, focusing on the rejected emails that include details such as sender address, email origin, and SMTP response codes.