DL is not receiving emails from external address.

Hi Team,

We have one particular DL not receiving email from external addresses.Internal emails are getting delivered.

we are getting this error when checked from Connectivity analyser.

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Attempting to send a test email message to informatica@xyz.test using MX mx record name.
 Delivery of the test email message failed.
 
Additional Details
 
The server returned status code 451 - Error in processing. The server response was: 4.7.1 Service unavailable - try again later
Exception details:
Message: Error in processing. The server response was: 4.7.1 Service unavailable - try again later
Type: System.Net.Mail.SmtpException
Stack trace:
at System.Net.Mail.RecipientCommand.CheckResponse(SmtpStatusCode statusCode, String response)
at System.Net.Mail.RecipientCommand.Send(SmtpConnection conn, String to, String& response)
at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception)
at System.Net.Mail.SmtpClient.Send(MailMessage message)
at Microsoft.Exchange.Tools.ExRca.Tests.SmtpMessageTest.PerformTestReally()
Elapsed Time: 766 ms.

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we are getting this error when checked fromGmail.

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The error that the other server returned was:
550 5.2.0 mailbox unavailable.

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we have checked all settings for this DL, external delivery is also enabled.

Please assist.

Thanks

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Abhishek Saxena