Bounces vs. new messages

Over the past year, Charter (may they all rot in purgatory) has begun
blocking outbound TMDA challenges with "relay not allowed" messages. So,
this is really a set of questions:

  - Is TMDA issuing challenges as bounces?
  - Is it possible to reconfigure TMDA to issue challenges as reply
    messages rather than bounces?

For the curious, I've found this perlism useful for spotting these
messages in my mail logs:

    sudo perl -ne '/(?<=550 relaying mail to ).*?(?=is not allowed)/ &&
	print "$&\n"' /var/log/mail.log{,.0} | sort -u

but TMDA doesn't really have any insight into the logs, and so this is
mostly informational for me. Perhaps it will be helpful to others as
well.

-- 
"Oh, look: rocks!"
	-- Doctor Who, "Destiny of the Daleks"