Question : A critical function required for the transfer of delivery of the message was not supported by the originator of the report

We had a user email a client they email on a regular basis.  Never before has the user had a problem e-mailing the client.  There were no attachments in the sent email.  The user recieved an undeliverable message; see below.

'John Doe' on 6/23/2010 8:40 AM
A critical function required for the transfer or delivery of the message was not supported by the originator of the report
<ournmailserver.com #5.3.3 smtp;500 5.3.3 Unrecognized command>

I'm guessing this is likely an issue on the clients side due to the fact we have not been expiriencing any email issues and have not made any changes to the envronment.  Any ideas why the undeliverable message has occured?

Thank you

Robert

Answer : A critical function required for the transfer of delivery of the message was not supported by the originator of the report

Based on NDR (ournmailserver.com #5.3.3 smtp;500 5.3.3 Unrecognized command) it looks your Exchange server is not able to communicate with remote email system.

1> have you observed any such issues with any other outgoing emails?
2> if no - this problem is with remote email system which you can only inform them and wait till it gets fixed.
3> if yes - you need to check what changes you have done in your exchange server recently.