we have exchnage2003 and 2007 for three years and never had the issue.
this is what happened lastnight
user from exchnage2003 sent an email to user on exchnage2007 and got the following error.
A critical function required for the transfer or delivery of the message was not supported by the originator of the report
5.3.3 smtp;500 5.3.3 Unrecognized command>
user from exchnage2007 to 2007 was able to send each others.
i looked the event viewer and services and found nothing to show any error.
i had to reboot the exchnage2007 and was able to send mail each others.