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When you create a distribution group, the default configuration is to reject messages from senders who are not authenticated. This means that users outside of your organization will not be able to send messages to your distribution groups. Generally, this is the desired configuration, but if needed, you can modify this setting on a distribution group to accept messages from external users using the following syntax:

    Set-DistributionGroup -Identity HelpDesk `
    -RequireSenderAuthenticationEnabled $false  

You can see here that we've disabled sender authentication for the HelpDesk distribution group. You can re-enable it at any time by setting the previous parameter value to $true.