M365EXO-023: External sender warning implemented (MS.EXO.7.1)
- Platform
- Entra ID / M365
- Category
- Advanced Threat Protection
- Severity
- Medium
- Zero Trust pillar
- Applications & Workloads (weight 1)
- Golden fixtures
- 3
- Branch coverage
- Observed: fixtures prove the verdicts they exercise
- Provenance
- baseline
What it checks
SCuBA MS.EXO.7.1 requires that external sender warnings be implemented. Marking mail that originates outside the organization helps users recognize impersonation and social engineering attempts. This can be delivered via the native external sender tag (Set-ExternalInOutlook) or a mail flow rule that prepends an indicator such as [External] to the subject of inbound external mail.
Recommended value
Native external sender identification enabled (Get-ExternalInOutlook Enabled = True) or a mail flow rule that prepends an external marker to inbound external mail
Remediation
Enable the native external sender identification feature so Outlook displays an External tag on mail from outside the organization. Alternatively, or in addition, create an enabled mail flow rule that prepends a marker such as [External] to the subject of mail received from outside the organization. Communicate the change to users so they understand what the indicator means.
Fixture-proven verdicts
Every verdict below is proven by a golden fixture in the module's gating test suite. This table derives from the last green run; it cannot be edited by hand.
| Scenario | Expected verdict |
|---|---|
| clean | PASS |
| known-bad | FAIL |
| throttled | Not Assessed |
Framework mappings
- CISA SCuBA
- MS.EXO.7.1v1
- NIST SP 800-53
- SI-8
- MITRE ATT&CK
- T1566