Limiting anonymous relay to distribution groups?

Exchange 2013

We've got an anonymous email relay that needs access to send to a distribution group.  The relay works fine for individual addresses, and we'd rather not permit access to the group for all external email addresses.  

I can create a contact for the sender's address and permit it to send to the group, but doing so requires I specify every address that can send to the group, which is not manageable.  One possible solution would be to create new groups specifically for relaying, but I'm hoping for something a little easier to implement and manage.

Anyone aware of an alternative?  Any chance it's possible to permit every address from our domain (i.e. *@abc.com)?

Thanks in advance for any suggestions.