The header contains X-Proofpoint-Spam-Details: rule=spam policy=default ... spamscore=89 ... reason=mlx, which is the Proofpoint spam engine verdict (MLX classifier) and indicates quarantine was driven by the spam policy evaluation, not by anti-virus or a user block list. In Proofpoint PPS/PoD, quarantine decisions frequently include an “X-Proofpoint-*Details” header that records the policy, rule family, and scoring components used to reach the final disposition. Here, the high spamscore=89 is decisive, and there is also an MLX log score entry supporting the ML-based spam classification. Antivirus-related quarantines typically show explicit malware/virus condemnation outcomes (e.g., malware score, “virus” rule, or attachment verdicts), while personal block list actions would be reflected as user-specific allow/block triggers, not the spam classifier rule. For IR triage, this header is the fastest way to validate why a message was quarantined and whether a false positive should be addressed by tuning spam thresholds, allow lists, or MLX-related settings rather than malware policies.