# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-M365-MAIL-007
title: Turn on MailTips, including external recipient warnings
finding:
  title: Outlook does not warn users before they send to external or large audiences
  description: Users get no hint when a reply goes outside the organization or to a very large group, which leads to accidental disclosure.
status: active
profiles: [baseline]
risk:
  severity: low
assessment: manual
tags: [exchange-online, outlook, mailtips, data-leakage, user-awareness]

description: |
  MailTips are enabled for the organization with these settings:

  - `MailTipsAllTipsEnabled`: `$true`;
  - `MailTipsExternalRecipientsTipsEnabled`: `$true` (warning when a recipient is outside the
    organization);
  - `MailTipsGroupMetricsEnabled`: `$true` (group size known to Outlook);
  - `MailTipsLargeAudienceThreshold`: at most {{ large_audience_threshold }} recipients.
rationale: |
  MailTips show a banner while the user writes the message, before it is sent. The external
  recipient tip catches the classic mistake of auto-completing a personal or partner address, and
  the large audience tip catches replies to a whole distribution list. Both are cheap ways to reduce
  accidental data leaks.
impact: |
  Users see more banners when writing to external or large audiences. No message is blocked.

parameters:
  - name: large_audience_threshold
    type: integer
    description: Number of recipients above which Outlook shows the large audience MailTip.
    unit: recipients
    default: 25
    minimum: 1
    maximum: 1000

audit:
  manual: |
    1. These settings are not shown in the admin centers. Connect to Exchange Online PowerShell as
       at least a Global Reader.
    2. Run (read-only):
       ```powershell
       Get-OrganizationConfig | Format-List MailTips*
       ```
    3. Fail if `MailTipsAllTipsEnabled`, `MailTipsExternalRecipientsTipsEnabled` or
       `MailTipsGroupMetricsEnabled` is `False`, or if `MailTipsLargeAudienceThreshold` is greater
       than {{ large_audience_threshold }}.

remediation:
  manual: |
    1. Connect to Exchange Online PowerShell as an Exchange Administrator.
    2. Run:
       ```powershell
       Set-OrganizationConfig -MailTipsAllTipsEnabled $true -MailTipsExternalRecipientsTipsEnabled $true `
         -MailTipsGroupMetricsEnabled $true -MailTipsLargeAudienceThreshold 25
       ```
       Replace `25` with the chosen threshold.
  rollback: |
    `Set-OrganizationConfig -MailTipsExternalRecipientsTipsEnabled $false`.

references:
  - title: MailTips in Exchange Online
    url: https://learn.microsoft.com/exchange/clients-and-mobile-in-exchange-online/mailtips/mailtips
    publisher: Microsoft
    kind: vendor-doc
    accessed: 2026-10-24
  - title: Set-OrganizationConfig
    url: https://learn.microsoft.com/powershell/module/exchangepowershell/set-organizationconfig
    publisher: Microsoft
    kind: vendor-doc
    accessed: 2026-10-24
  - title: Maester MT.1039
    url: https://maester.dev/docs/tests/MT.1039
    publisher: Maester
    kind: tool
    accessed: 2026-10-24
mappings:
  - framework: cis-m365
    framework_version: "7.0.0"
    id: "6.5.2"
    relation: equivalent
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-4
    relation: related
related_controls:
  - id: CB-M365-MAIL-003
    type: related

authors:
  - name: Alice
    github: Sn0wAlice
introduced_in: 0.1.0
created: 2026-10-24
updated: 2026-10-24