Hi,

I am not sure if this is what is causing our issue , but 3 weeks ago one of our admins enabled the rejection of unknown users on our exchange.

Right now when we create a new exchange distribution group, and try to send an email from outside to the group, we receive an error:

550 5.1.1 User unknown.

Anything I should be looking at to troubleshoot this issue?

Thank you for your help.

This is the default behavior of  any mail universal Distribution group

When created it will not allow you to receive emails from external domains

Perform the below to resolve this 

Go to the properties of the  new distribution group that you created - select mail flow settings - select message delivery restrictions and uncheck the option that require that all senders are authenticated

Hi Sathish,

Thank you for your answer.

However, I am unable to find what you are talking about.

I am not sure we are talking about same version, but we are using Exchange 2013.

When I go to properties of the new group, I don't have 'mail flow settings' , usually we go to delivery management and we check the option 'Senders Inside and outside of my organization'.

Where is the setting you are talking about?

I found my issue.

It was the credential service that was not running on the edge server.

After starting that service, the synchronization worked fine and then no more unknown user.

Thank you.