The process of creating a distribution list and making it receive email from all senders (local and external) wasn’t a big deal in Exchange 2003. But things changed in Exchange 2007 and 2010, causing external senders to get this error:
#< #5.7.1 SMTP; 550 5.7.1 RESOLVER.RST.AuthRequired; authentication required>
By default, Exchange 2007 and 2010 allow only authenticated users to send to distribution lists. To change this behavior, use one of the following methods:
1- Run this command from the Exchange Management Shell (EMS) to disable the option Require that all senders are authenticated
Set-DistributionGroup_Your_Distribution_List -RequireSenderAuthenticationEnabled $False
Or
2- Open the Exchange Management Console (EMC)
- Locate the distribution list in question
- Right-click on it and select Properties
- Open the Mail Flow Settings tab
- Select Message Delivery Restrictions
- Open the Properties tab
- In the Message Delivery Restrictions window, disable the option Require that all senders are authenticated
