<div>
Looks like it's an invalid email address. &nbsp;Log into that mailbox and send yourself an email then reply to that email.
</div>


<div>
email address is correct. I see in properties one box checked , I unchecked -require all senders to be authenticated.<br />
<br />
but checking or unchecking how it will help<br />
<a href="https://filedb.experts-exchange.com/incoming/2018/04_w14/1293857/Capture.PNG" target="_blank" class="file-inline" title="sender (5 KB)"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path d="M67.508 468.467c-58.005-58.013-58.016-151.92 0-209.943l225.011-225.04c44.643-44.645 117.279-44.645 161.92 0 44.743 44.749 44.753 117.186 0 161.944l-189.465 189.49c-31.41 31.413-82.518 31.412-113.926.001-31.479-31.482-31.49-82.453 0-113.944L311.51 110.491c4.687-4.687 12.286-4.687 16.972 0l16.967 16.971c4.685 4.686 4.685 12.283 0 16.969L184.983 304.917c-12.724 12.724-12.73 33.328 0 46.058 12.696 12.697 33.356 12.699 46.054-.001l189.465-189.489c25.987-25.989 25.994-68.06.001-94.056-25.931-25.934-68.119-25.932-94.049 0l-225.01 225.039c-39.249 39.252-39.258 102.795-.001 142.057 39.285 39.29 102.885 39.287 142.162-.028A739446.174 739446.174 0 0 1 439.497 238.49c4.686-4.687 12.282-4.684 16.969.004l16.967 16.971c4.685 4.686 4.689 12.279.004 16.965a755654.128 755654.128 0 0 0-195.881 195.996c-58.034 58.092-152.004 58.093-210.048.041z"/></svg>Capture.PNG</a>
</div>


Capture.PNG

<div>
The error clear shows that their is an issue with the recipient address. Are you attempting to send or receive the email.
</div>


<div>
Can you please specify when you getting this error?<br />
Are you using Outlook to send email to this address or this is web based form ?<br />
<br />
It looks like RCPT command was not sent after Message From:<br />
<br />
Try to use telnet and check all server responses.
</div>


<div>
Looking at your error message the email address is spelled two different ways. Are you SURE that it's not a typo?
</div>


<div>
I unchecked this box, it went through now ( require that all senders are authenticated)<br />
<br />
I was sending email from 1 domain to another<br />
<a href="https://filedb.experts-exchange.com/incoming/2018/04_w14/1293878/Capture.PNG" target="_blank" class="file-inline" title="Capture.PNG (5 KB)"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path d="M67.508 468.467c-58.005-58.013-58.016-151.92 0-209.943l225.011-225.04c44.643-44.645 117.279-44.645 161.92 0 44.743 44.749 44.753 117.186 0 161.944l-189.465 189.49c-31.41 31.413-82.518 31.412-113.926.001-31.479-31.482-31.49-82.453 0-113.944L311.51 110.491c4.687-4.687 12.286-4.687 16.972 0l16.967 16.971c4.685 4.686 4.685 12.283 0 16.969L184.983 304.917c-12.724 12.724-12.73 33.328 0 46.058 12.696 12.697 33.356 12.699 46.054-.001l189.465-189.489c25.987-25.989 25.994-68.06.001-94.056-25.931-25.934-68.119-25.932-94.049 0l-225.01 225.039c-39.249 39.252-39.258 102.795-.001 142.057 39.285 39.29 102.885 39.287 142.162-.028A739446.174 739446.174 0 0 1 439.497 238.49c4.686-4.687 12.282-4.684 16.969.004l16.967 16.971c4.685 4.686 4.689 12.279.004 16.965a755654.128 755654.128 0 0 0-195.881 195.996c-58.034 58.092-152.004 58.093-210.048.041z"/></svg>Capture.PNG</a>
</div>


pramod1

<div>
<a href="https://technet.microsoft.com/en-us/library/bb124405(v=exchg.141).aspx" rel="ugc">https://technet.microsoft.com/en-us/library/bb124405(v=exchg.141).aspx</a>
</div>


<div>
<div class="content wysiwyg-content">
hi<br />
<br />
I am getting below error.<br />
<br />
Subject: Undeliverable: xxxx<br />
<br />
The original message was received at Mon, 2 Apr 2018 19:15:53 GMT from m0121180.xxx[127.0.0.1]<br />
<br />
&nbsp; &nbsp;----- The following addresses had permanent fatal errors ----- &lt;imdoamininframanagement@d<wbr />omain.com&gt;<wbr /><br />
&nbsp; &nbsp; (reason: 550 5.1.1 User unknown)<br />
<br />
&nbsp; &nbsp;----- Transcript of session follows ----- ... while talking to [216.x.x.x]:<br />
&gt;&gt;&gt;&nbsp;DATA<br />
&lt;&lt;&lt;&nbsp;550 5.1.1 User unknown<br />
550 5.1.1 &lt;imdomaininframanagement@d<wbr />omain.com&gt;<wbr />... User unknown &lt;&lt;&lt;&nbsp;503 5.5.2 Need rcpt command<br />
<br />
isit due to restriction for large DLs. &nbsp;<br />
::DISCLAIMER::
</div>
</div>


hi

I am getting below error.

Subject: Undeliverable: xxxx

The original message was received at Mon, 2 Apr 2018 19:15:53 GMT from m0121180.xxx[127.0.0.1]

   ----- The following addresses had permanent fatal errors ----- <imdoamininframanagement@domain.com>
    (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 <imdomaininframanagement@domain.com>... User unknown <<< 503 5.5.2 Need rcpt command

isit due to restriction for large DLs.  
::DISCLAIMER::

exchange

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.