DOCUMENT:Q280118  11-APR-2001  [exchange]
TITLE   :XFOR: Non-Delivery Report with "501 Sender Domain Must Exist" Me
PRODUCT :Microsoft Exchange
PROD/VER::5.5
OPER/SYS:
KEYWORDS:

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The information in this article applies to:

 - Microsoft Exchange Server, version 5.5 
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SUMMARY
=======

Some Microsoft Exchange e-mail messages generate a non-delivery report (NDR)
with the message "501 sender domain must exist." This behavior occurs because
the sender domain is not registered on the Internet. Check with your Internet
service provider (ISP) or Domain Name System (DNS) administrator to verify that
the sender's domain has MX records on the Internet.

MORE INFORMATION
================

Some other (non-Exchange Server) mail systems check the sender's e-mail domain
to prevent junk e-mail (spam). This is called performing a reverse lookup.

When these mail systems receive an SMTP connection, they attempt name resolution
based on the address in the From field. If the address in the From field cannot
be resolved by DNS, the receiving mail system responds with "501 Sender domain
must exist," and closes the connection, causing the NDR.

As a test, try to telnet the destination server over port 25. You receive the
following results

   mail from:test@DomainTest.com
   501 test@DomainTest.com... Sender domain must exist

and the connection generates the NDR.

Then, try opening a telnet session over port 25 from a domain that is known to
work. The following results are returned:

   mail from:test@Microsoft.com
   250 test@Microsoft.com... Sender ok

Additional query words:

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Keywords          :  
Technology        : kbExchangeSearch kbExchange550 kbZNotKeyword2
Version           : :5.5
Issue type        : kbinfo

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