Your message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you”

Hi

I have a distribution group called "marketing" in our outlook address book and there are 2 members within this .If I send an email within our organisation both of the members receive the email.

But another external organisation is sending email from their domain to the same distribution group marketing and they are getting a message that says:
“Your message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you”
Not sure if an external organisation can send an email to a distribution group on our organisation address book.
Any help to troubleshoot this error will be great.
Thanks
lianne143Asked:
Who is Participating?
 
Pete LongTechnical ConsultantCommented:
By default distribution groups are not available from 'Outside', you need to manually enable the group for external access.

>>properties -> mail flow settings -> uncheck the "require that all senders are authenticated"

Or on Exchange 2013/2016 Properties of the group > delivery management > Allow internal and external users.
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