Restrict Users from Receiving Email Outside the Organization

Objective

Prevent specific users (or all users) in your organization from receiving emails sent from external domains (outside your Microsoft 365 tenant).

Prerequisites

• You must be assigned the Exchange Administrator or Global Administrator role in Microsoft 365.

Steps to Configure the Restriction

1. Access the Exchange Admin Center

• Go to: https://admin.exchange.microsoft.com
• Sign in with your admin credentials.

2. Navigate to Mail Flow Rules

• In the left-hand navigation pane, select Mail flow.
• Click on Rules.

3. Create a New Rule

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

4. Configure the Rule Settings

• Name the rule something descriptive, e.g., Block External Emails to [User/Group].

• Under Apply this rule if…:

  • Select The recipient is → choose the specific user(s) or group(s).
  • Add another condition: The sender is located → select Outside the organization.

• Under Do the following…:

  • Choose Reject the message with the explanation.
  • Enter a custom message, e.g.,
    "This recipient is not authorized to receive emails from external senders."

• Leave other settings as default unless specific customization is needed.

5. Set Rule Mode

• Under Choose a mode for this rule, select Enforce.

6. Save and Apply

• Click Next, review the configuration, and click Finish to activate the rule.

Optional: Add Exceptions

If you want to allow certain external domains or addresses, you can:

• Add an exception under “Except if…” such as:
  • The sender's domain is → specify trusted domains (e.g., partnerdomain.com).

Testing the Rule

• Send a test email from an external address to the restricted user.
• The message should be rejected with the custom explanation you configured.

Notes

• This rule only affects inbound external emails. Internal mail flow remains unaffected.
• You can modify or disable the rule at any time from the Mail flow > Rules section.