I have a listmember who has just started having problems posting to the
list.  They get the following message back:

The original message was received at Thu, 16 Oct 1997 00:57:08 -0400
from port113.clea2.prodigy.net [204.237.92.113]

   ----- The following addresses had permanent fatal errors -----
<list address>

   ----- Transcript of session follows -----
... while talking to list host:
>>> MAIL From:<[log in to unmask]> SIZE=404
<<< 571 <[log in to unmask]>... This domain is banned.
554 <list address>... Service unavailable

     Could someone explain what '571....This domain is banned' means?  How
do I help them?
     Much appreciated,

     Tea