An external user sends an email to your distribution group like email@example.com, but gets a bounced message back.
Your message wasn’t delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.
By default, distribution groups in Exchange 2007 Server are secure. This means that only internal AD users can send an email to a distribution group, even if the group has a proper email address (externally accessible).
Navigate to EMC -> group properties -> mail flow settings tab -> delivery restriction, you will see a check box next to “require all users are authenticated”. Uncheck the box, so that external users can send email to the group.
Set-DistributionGroup -Identity groupname -RequireSenderAuthenticationEnabled:$false
If you want to do the same for all your distribution groups, run the shell command
Get-DistributionGroup | Set-DistributionGroup -RequireSenderAuthenticationEnabled:$false