troubleshooting Question

How do I troubleshoot this error: There was a SMTP communication problem with the recipient's email server?

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JamesAdmin asked on
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We have our own Exchange 2003 running on Windows SBS 2003 and having a problem sending to one domain in particular.
When I send e-mail to the domain in question, I get:
There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator. < #5.5.0 smtp;550-Verification failed for>>
I did a reverse DNS lookup and originally it was pointing to (myip = our external static ip)
I called my ISP and had them change the reverse DNS (PTR record) to:
myip PTR record: [TTL 600s] [A=myip]
When I telnet to their email server I get: ESMTP Exim 4.66 #1 Tue, 14 Aug 2007 08:02:49 -0700
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.
250 Hello hidden-user at  [myip]
250 Hello hidden-user at [myip]
250 OK
550-Verification failed for <>
550-The mail server could not deliver mail to
The account or domain may not exist, they may be blacklisted, or missing the pro
per dns entries.
550 Sender verify failed
Also the user I'm trying to send to is getting the same error when he tries to send to me
One more comment, when I telnet to my email server from outside the office I get
250 Hello [] = is my firewall/gateway
Anyone have any idea where to go from here?

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