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?