rblsmtpd 451 Sender has sent to LashBack

Bounce message received:

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

[email protected]

Malformed SMTP reply from mail.server.com [192168.1.1]
in response to initial connection:
rblsmtpd: pid 14483: 451 Sender has sent to LashBack
Unsubscribe Probe accounts:
retry timeout exceeded

Looking further into this it appears that the sending IP address was listed in a RBL black list.  The RBL server being queried was: ubl.unsubscore.com

This in turn (www.unsubscore.com) took me to: http://www.lashback.com/support/UnsubscribeBlacklistSupport.aspx

Most RBL systems return a useful message with a URL, but, not lashback.