Send & Receive Issues

How to Create an Exchange Transport Rule for Approved External Senders

Anum Mushtaq Oct 16, 2026 868 views

Question details

The user needs to configure mail flow rules to restrict external senders from emailing a Microsoft 365 mail-enabled group, allowing only specific approved external users.

How to Create an Exchange Transport Rule for Approved External Senders

Product: Microsoft Exchange
Device & OS: not provided
Scenario: Managing email delivery restrictions for a mail-enabled group to prevent spam or unauthorized external emails while maintaining contact with select partners.
Observed behavior: The goal state is to have the group reject all external emails by default except for two specifically approved external email addresses.

Before you start

Ensure you have Exchange Administrator or Global Administrator privileges in your Microsoft 365 tenant to create and manage mail flow rules.

Solution 1Recommended

Configure a Mail Flow Rule in Exchange Admin Center

Create a transport rule that rejects external emails sent to the target group, while adding an exception for your approved external senders.

To successfully filter emails, the mail-enabled group must first be configured to accept external emails. If the group blocks external senders at the directory level, the transport rule will never be triggered because the emails will be dropped before reaching the rule processing phase.

Enable external senders for the group

Go to the Microsoft 365 Admin Center, navigate to 'Teams & groups' > 'Active teams and groups'. Select your group, click the 'Settings' tab, and check the box for 'Allow external senders to email this group'.

Access the Exchange Admin Center

Open the Exchange Admin Center (EAC). On the left-hand navigation menu, click on 'Mail flow' and then select 'Rules'.

Create a new transport rule

Click the '+' icon (Add a rule) and select 'Create a new rule'. Provide a descriptive name for the rule, such as 'Restrict External Emails to Group'.

Define the conditions

Under 'Apply this rule if', click 'Add condition'. Select 'The recipient is' and choose your target group. Add a second condition: select 'The sender is located' and choose 'Outside the organization'.

Set the block action

Under 'Do the following', select 'Block the message' and choose 'reject the message and include an explanation'. You can enter a custom rejection message for unapproved senders.

Add the approved sender exceptions

Under 'Except if', click 'Add exception'. Select 'The sender is' and manually enter the email addresses of the two approved external senders. Save the rule and test it.

Configure a Mail Flow Rule in Exchange Admin Center

Rule Propagation Time: New Exchange transport rules may take up to 30 minutes to fully propagate and take effect across your Microsoft 365 environment.

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Frequently Asked Questions

Why are my approved external senders still getting rejected?

Ensure that the mail-enabled group itself is configured to allow external senders. If the group settings block external emails by default, the Exchange transport rule will not override this group-level restriction.

Can I apply this transport rule to multiple groups at once?

Yes. In the Exchange Admin Center, under the 'The recipient is' condition, you can select multiple mail-enabled groups or distribution lists to apply the same restriction and exceptions to all of them.

How can I test the transport rule without interrupting actual mail flow?

You can set the rule's mode to 'Test with Policy Tips' or 'Test without Policy Tips' in the Exchange Admin Center. This allows you to evaluate the rule's impact via message tracking logs before enforcing it across your organization.

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