Mail Flow Issue - 503 5.5.1 Error: send HELO/EHLO first

Our users are receiving 550 4.4.7 QUEUE.Expired undeliverable messages only to a single domain.

Using the Troubleshooting Assistant, the following Mail Flow issue is found:
Remote server mail.domain.com failed the mail acceptance test. MAIL FROM command: Respond = 503 5.5.1 Error: send HELO/EHLO first.

We confirmed all of the messages sent to this domain are stuck in the queue.  No configuration changes have been made on the local Exchange server or on the remote mail server, which happens to be Postfix.
gatorITAsked:
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George SasConnect With a Mentor IT EngineerCommented:
I still believe this is a Postfix issue and authentication between the 2 systems.
One of the systems , your or the postfix is asking for authentication.
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George SasIT EngineerCommented:
Well , I am sure someone PATCHED the Postfix on the other end.
Try to talk with them and have them check the system and what the patch did.

Had this issue a few years ago with an internal Postfix (I am not a postfix man) and the guy responsible for the system patched postfix and then the error started.

Check on your exchange if you have enabled TSL on the smtp send connector and try disabling that first.
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George SasIT EngineerCommented:
Btw , try to send a mail to that system using telnet and see if you are able to send it that way.
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gatorITAuthor Commented:
TLS is not enabled on the send connector and I have tried sending via telnet which was successful.
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George SasIT EngineerCommented:
What exchange version are you running ?
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gatorITAuthor Commented:
2007
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George SasIT EngineerCommented:
And , are you using an FQDN on the send connector ? something that can be solved from outside also ?
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gatorITAuthor Commented:
Interestingly enough I am trying to send from another Exchange 2007 mail server (completely separate and unrelated) and the mail is queued there as well.  
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gatorITAuthor Commented:
Yes, I verified in the smtpsend logs that it's sending the FQDN.  The EHLO is being sent as well, so not sure why Postfix isn't seeing it that way.
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George SasIT EngineerCommented:
Are you using any gateway to send the mails out or do they go out DIRECTLY from your exchange ?
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gatorITAuthor Commented:
Like a smart host?  No, just DNS.  We have a smarthost available that I was going to try.
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George SasIT EngineerCommented:
I would contact the admin on the other side asap and ask him to look trough his logs and see what the problem is and what message does he see coming from your end when you try to submit mail.
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gatorITAuthor Commented:
Due to time constraints we implemented a smart host for this domain that resolved the issue.  Mentioned to the remote mail server admin that they might research further on their end.  
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George SasIT EngineerCommented:
I think you did all you could on your end ... but still 100% sure the error is on the receiver side
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