Question : remote host "128.121.4.11", responded to the SMTP command "rcpt" with 550 5.7.1 Unable to relay

We are having an issue sending mail to the domain castigliaeaton.com.   Most of the time the mail goes through without issue.  Occasionally, an NDR is generated and the mail doesn't go through.  The error below corresponds the the time of the NDR.  

One odd thing I noticed while researching the issue is that the address 128.121.4.11 is NOT there mail server.  The MX record for castigliaeaton.com points to 128.121.4.6.  I have verified this with there provider however they were not able to provide any assistance beyond that.

I thought it could be some kind of DNS issue so I changed our DNS forwarders over to 4.2.2.1 and 4.2.2.2 temporarily while troubleshooting with Microsoft.

Any assistance is greatly appreciated.


Event Type:      Error
Event Source:      MSExchangeTransport
Event Category:      SMTP Protocol
Event ID:      7004
Date:            3/1/2007
Time:            10:39:17 AM
User:            N/A
Computer:      OURSERVER
Description:
This is an SMTP protocol error log for virtual server ID 1,
connection #94. The remote host "128.121.4.11", responded to
the SMTP command "rcpt" with "550 5.7.1 Unable to relay for
[email protected]  ". The full command sent was
"RCPT TO:com>  ".  This will probably
cause the connection to fail.

For more information,
click http://www.microsoft.com/contentredirect.asp.


We are running SBS 2003 with Exchange 2003 SP2

Answer : remote host "128.121.4.11", responded to the SMTP command "rcpt" with 550 5.7.1 Unable to relay

Yes that may be. But the problem is definitely on their side. The host 128.121.4.11 does also run an external SMTP server:

Trying 128.121.4.11...
Connected to 128.121.4.11.
Escape character is '^]'.
220 win2018.nexpoint.net Microsoft ESMTP MAIL Service, Version: 5.0.2195.6713 ready at  Tue, 6 Mar 2007 02:23:29 -0500
quit
221 2.0.0 win2018.nexpoint.net Service closing transmission channel
Connection closed by foreign host.

Maybe their provider is just testing something and so sometimes e-mail works fine and sometimes it does not.
Or they have a misconfiguration of some kind.