This is mainly addressed to the IronPort employees who hang out here.
I found this thread on a discussion board while searching for something else. The bottom line is that AsyncOS does not follow RFC 2034 in its use of enhanced SMTP status codes because it precedes them with a "#", like this:
550 #5.1.0 Address rejected.
instead of this:
550 5.1.0 Address rejected.
This prevents SMTP clients which implement RFC 2034 from noticing the enhanced status code. AsyncOS doesn't advertise the ENHANCEDSTATUSCODES capability, so it isn't actually breaking the rules. But if IronPort is going to go to the effort of putting those codes in there, then why not follow RFC 2034?