Delivery has failed to these recipients or distribution lists

Delivery has failed to these recipients or distribution lists:
Your message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.

Diagnostic information for administrators:
Generating server:
#< #5.7.1 smtp;550 5.7.1 RESOLVER.RST.AuthRequired; authentication required> #SMTP#

Dear Experts,

How do I resolve this?
Anonymous KHIT EngineerAsked:
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Zacharia KurianAdministrator- Data Center & NetworkCommented:
This is because the Distribution Group/Mailbox named ""  requires "all senders to be authenticated".
In your  Exchange Management Console;
Click on the properties of   Distribution Group/Mailbox named ""
         Click the “Mail Flow Settings” Tab
         Click the “Message Delivery Restrictions” item
          Click the “Properties…” button
          Uncheck the “Require that all senders are authenticated” check-box
          Click “OK” button
        Click “OK” button again


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Anonymous KHIT EngineerAuthor Commented:
Hi! Zac,

This the recipient's email address, not ours.
Anonymous KHIT EngineerAuthor Commented:
What is the cause of this message to pop-up?

Previously, when the user sent an email to them, there was no issue.
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Zacharia KurianAdministrator- Data Center & NetworkCommented:
Then you need to check with them if the mailbox is been restricted or not. If so they need uncheck "Requires all senders to be authenticated".

Anonymous KHIT EngineerAuthor Commented:
Hi! R-R,

Thanks but this issue lies with the recipient.

Is there anything the sender side can do or what is causing this issue in the first place as it never happened before.
Anonymous KHIT EngineerAuthor Commented:
This is an authentication issue only is it?
Zacharia KurianAdministrator- Data Center & NetworkCommented:
As per the NDR you have received, yes.

Please try to send email with another user and if the same issue occurs.
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