Syntax error, command unrecognized

I GOT THE FOLLOWING MESSAGE , when i send a mail to my client , their mail server reply this mail to me , Would anyone know what happen is it ?

My mail config:
NT 4 SP6
EXCHANGE 5.5

Your message did not reach some or all of the intendedrecipients.

Subject: Mail test Sent:11/6/2003 11:27

The following recipient(s) could not be reached:

support@sea.com on 11/6/2003 11:40
Unable to deliver the message due to a communications failure
The MTS-ID ofthe original message is: c=xx;a= ;p=xxxxxx;l=xxx-0306242338Z-2345
MSEXCH:IMS:xxxxx:xxxx:xxxxx 3500 (000B09AA) 500 Syntax error, command unrecognized