"Unrecognized Command" message when replying to email

One of our users is getting the following error message when replying to emails from a certain client of ours.  To my knowledge, no one in our company has ever gotten one of these, and it seems to be isolated to replies to this one domain.  New emails to the same address, for instance, do not generate this bounce message.

          A critical function required for the transfer or delivery of the message
          was not supported by the originator of the report
            <mail.ourdomain.com #5.3.3 smtp;500 5.3.3 Unrecognized command>

I've searched Experts-Exchange and found a very similar problem at this link:  http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/Q_23196362.html

However, the resolution for that case was to turn off client side email security software, and we have none.  

Any other ideas?

witzph1Asked:
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MesthaConnect With a Mentor Commented:
That would tend to point to the remote site that has the problem. However I have seen it when SMTP scanning is on firewalls, Cisco in particular.
Its an SMTP error, so the problem is somewhere between your server and theirs.

Simon.
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witzph1Author Commented:
I am going to tend to agree with you that the problem is on the other end, but I wanted a second opinion.  We have no SMTP scanning on our firewall that I know of.  Just the fact that we've never gotten this message before, and we get no errors emailing new messages to the domain, and we've never gotten any of these error messages when replying to any other domain, points everything to the other server.

Thanks for your help.
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