Hoople - 5:18 pm on Apr 18, 2012 (gmt 0)
Could you post the full error? Obfuscate the domain plz.
What might help is turning up logging on the receive connector on your Hub Transport server.
The 'Bad Sequence of Commands' sounds like a sending server not following RFC spec. Might be a java/php script passing on the content of a form or SPAM.
I'm an Exchange guy by day, webmaster at night :-)