Distribution Lists Open to External Senders
The question it answers: Which distribution lists and Microsoft 365 groups accept mail from anyone on the internet, and how many people do they reach?
A large distribution list that accepts mail from anyone on the internet turns one phishing mail into a company-wide campaign - and bypasses the per-recipient friction attackers usually face. Lists that genuinely need outside mail (support@, info@) are fine; ‘All staff’ is not.
At a glance
Section titled “At a glance”| Where in the app | Exchange Online › Mail Security Posture › Lists Open to External Senders |
| Workload | Exchange Online |
| Kind | Table · dashboard tile |
| Data | Read from Microsoft 365 when someone first opens it, then kept for the report cache lifetime (Settings > Data & cache, 1 hour unless changed). It reads through PowerShell (Exchange Online or SharePoint Online), so the first load takes longer than a Graph report. |
| Licence | Exchange Online. |
| Report id (API and MCP) | exchange.posture.open-mail-groups |
Columns
Section titled “Columns”| Column | Key | Type |
|---|---|---|
| Group | Name | Text |
| Address | Address | Text |
| Members | Members | Status |
| Type | GroupType | Text |
| Owners | ManagedBy | Text |
| Hidden from GAL | HiddenFromGal | YesNo |
What it needs
Section titled “What it needs”Licence: Exchange Online.
Application permissions on the IntensiveReports app registration (all permissions):
| Permission | API | What it allows | Note |
|---|---|---|---|
Exchange.ManageAsApp | Office 365 Exchange Online | Manage Exchange As Application |
Data sources it reads
Get-DistributionGroupGet-DistributionGroupMemberGet-UnifiedGroup
Through the API and MCP
Section titled “Through the API and MCP”GET /ir-api/v1/reports/exchange.posture.open-mail-groups?size=100An AI assistant reads it with the MCP tool get_report and the id exchange.posture.open-mail-groups. The client needs the Read grant for Exchange Online. See Using the API.