Require that all senders are authenticated

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This option is available in the Online Control Panel on the Mail Flow Settings tab of a Mailbox or a Distribution List.

If you configure a mailbox or distribution list to require that all senders are authenticated, any messages from senders that do not have valid logon credentials in your organization will be rejected. By default, only new distribution lists are configured to require all senders to be authenticated.

Use this restriction to specify that recipients receive messages only from internal senders that have been successfully authenticated. For example, to prevent messages that originate outside of your Exchange organization from being delivered to distribution groups that are used for internal communications, you can configure these groups to require sender authentication.