External email warning banner in O365:


? Step 1: Sign in to Exchange Admin Center

  1. Go to https://admin.exchange.microsoft.com.

  2. Log in with your Global Admin / Exchange Admin account.

? Step 2: Create a Mail Flow Rule

  1. In the left menu, select Mail flow > Rules.

  2. Click + Add a rule > Create a new rule.

  3. Give it a name, e.g., External Email Warning Banner.

? Step 3: Configure the Conditions

  • Under Apply this rule if… choose:

    • The sender is located… → Outside the organization

? Step 4: Configure the Action

  • Under Do the following… choose:

    • Prepend the message subject with…
      Example: [EXTERNAL]

    • AND/OR

    • Apply a disclaimer to the message… → Prepend a disclaimer

In the disclaimer text box, paste your warning banner. For example:

<div style="background-color:#ff0000; color:white; padding:10px; font-size:14px;"> ⚠️ This email originated from outside your organization. Do not click links or open attachments unless you recognize the sender.
</div>

  • Set the fallback action: Wrap.

? Step 5: Exclusions (Optional)

  • Add exceptions so your own trusted domains don’t trigger the banner.

  • Under Except if… choose:

    • The sender domain is… → add your internal domains (e.g., yourcompany.com).

? Step 6: Save and Test

  • Click Save.

  • Send a test email from a Gmail/Yahoo account to verify the red banner is displayed.