Sigma rule: Suspicious Email Delivered In Microsoft 365

Rule

Title

Suspicious Email Delivered In Microsoft 365

Status

experimental

Level

medium

Author

Marco Pedrinazzi (@pedrinazziM) (InTheCyber)

Date

2026-01-27

File

rules/cloud/m365/audit/microsoft365_suspicious_email_delivered.yml

Commit

ccbb0369507f796db0b0ae6854c8a2be471b9d84

Detects instances where an email, identified as malicious or suspicious by the Microsoft Defender for Office 365 (formerly ATP) engine, was delivered to a user's Inbox or Junk folder.
It might indicate that a potential threat, such as a spearphishing attachment or links, has bypassed initial blocking mechanisms and reached an end-user, requiring further investigation and potential remediation.

Log source

product

m365

service

audit

Detection

selection:
    Workload: 'ThreatIntelligence'
    Operation: 'TIMailData'
    Directionality: 'Inbound'
filter_main_blocked:
    DeliveryAction: 'Blocked'
condition: selection and not 1 of filter_main_*

False positives (as the rule lists them)

• Unlikely

Tags

• attack.initial-access
• attack.t1566.001
• attack.t1566.002

MITRE ATT&CK ids this rule names

• T1566.001
• T1566.002

References

• https://learn.microsoft.com/en-us/defender-office-365/threat-explorer-real-time-detections-about
• https://research.splunk.com/cloud/605cc93a-70e4-4ee3-9a3d-1a62e8c9b6c2/
• https://github.com/Bert-JanP/Hunting-Queries-Detection-Rules/blob/e7250648cb16d4a497ae8737943bf010ea96d2e6/Defender%20For%20Cloud%20Apps/MaliciousEmailDeliveredInMailbox.md

Read on 2026-11-03T00:58:21.399Z under licence sigma-drl-1.1@d5f4c36a from https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules/cloud/m365/audit/microsoft365_suspicious_email_delivered.yml