Exchange 2013 - Rejected Recipient(550 5.1.1

I'm troubleshooting an issue whereas a DL is set to allow external senders to send to it, however I receive this message:

Rejected Recipient(550 5.1.1 <testing@domain.com>: Recipient address rejected: User unknown in relay recipient table)
Scotch TechITAsked:
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Derek SouterITO Svc Delivery Cons IIICommented:
you need to make sure that the "Require that all senders are authenticated" check box is unticked on the distribution list properties
messagedeliveryrestrictions.png
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Derek SouterITO Svc Delivery Cons IIICommented:
as nobody else has commented, and the question appears abandoned, I am going to mark my response as the best - there doesn't seem to be any other option - personally I would leave it open forever, or until an admin of the site closed it.
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