I just started getting this in reply to any mail sent to a specific mailing list of our domain zzzz.com (the mail for which is handled by an external company):
<xxxx.yyyy@zzzz.com> (expanded from <mailinglist@zzzz.com>): host 10.2.3.30[10.2.3.30] said: 554 Service unavailable; Client host [10.2.2.20] blocked using list.dsbl.org (in reply to RCPT TO command)
So actually there is one of their internal servers who has banned another of their internal servers using a RBL.
Not sure if TRWTF is not listing the internal network as trusted/whitelisted or list.dsbl.org containing a RFC 1918 address block.