If a hacker gains access to a user's mailbox, they can auto-forward the user's email to an outside address and steal proprietary information. You can stop this by creating a mail flow rule.
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From the Microsoft 365 admin center, select Exchange, mail flow, and on the rules tab, select the plus sign and choose create a new rule.
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Select More options. Name your new rule.
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Then open the drop-down for apply this rule if, select the sender, and then is external internal.
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Select Inside the organization, and then OK.
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Choose add condition, open the drop-down, select The message properties, then include the message type.
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Open the select message type drop-down, choose Auto-forward, then OK.
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Open the Do the following drop-down, select Block the message, then reject the message and include an explanation.
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Enter the message text for your explanation, then select OK.
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Scroll to the bottom and select Save.
Your rule has been created, and hackers will no longer be able to auto-forward messages.
