When a SharePoint list is being email enabled the Central Administration service accounts creates a new contact object in Active Directory. The object attributes configured by SharePoint are fully sufficient for the Exchange transport routing engine when addressed directly. Which means that a new email sent to the email address of the email enabled list is routed to SharePoint.
However, when the contact has been added to an Exchange distribution list, the Exchange routing engine will not resolve the contact object. You won't see any information on this situation in the Exchange message tracking log.
The solution is pretty simple but require the intervention of an Exchange administrator. Execute the following on the mail contact object:
Get-Contact CONTACTNAME | Set-Contact
The use of Set-Contact without any additonal parameters forces Exchange to validate the currently exisiting attributes and the follow four attributes:
Now the mail contact object is a fully routable member or the Exchange organization.
Blame it on SharePoint, not Exchange.