I am getting below error.

Subject: Undeliverable: xxxx

The original message was received at Mon, 2 Apr 2018 19:15:53 GMT from[]

   ----- The following addresses had permanent fatal errors ----- <>
    (reason: 550 5.1.1 User unknown)

   ----- Transcript of session follows ----- ... while talking to [216.x.x.x]:
>>> DATA
<<< 550 5.1.1 User unknown
550 5.1.1 <>... User unknown <<< 503 5.5.2 Need rcpt command

isit due to restriction for large DLs.  
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Scott CSenior EngineerCommented:
Looks like it's an invalid email address.  Log into that mailbox and send yourself an email then reply to that email.
pramod1Author Commented:
email address is correct. I see in properties one box checked , I unchecked -require all senders to be authenticated.

but checking or unchecking how it will help
timgreen7077Exchange EngineerCommented:
The error clear shows that their is an issue with the recipient address. Are you attempting to send or receive the email.
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Tom CieslikIT EngineerCommented:
Can you please specify when you getting this error?
Are you using Outlook to send email to this address or this is web based form ?

It looks like RCPT command was not sent after Message From:

Try to use telnet and check all server responses.
William FulksSystems Analyst & WebmasterCommented:
Looking at your error message the email address is spelled two different ways. Are you SURE that it's not a typo?
pramod1Author Commented:
pramod1Author Commented:
I unchecked this box, it went through now ( require that all senders are authenticated)

I was sending email from 1 domain to another
William FulksSystems Analyst & WebmasterCommented:
OK, so that fixed it but you were wanting to know what that option does? It has to do with associating login credentials to the account. It's mostly used with distribution groups, but here's more info -

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