> While we're on the subject...can anyone explain this one (and perhaps, a
> solution?)
 
The error message is really pretty clear if you are familiar with the
terms used.  Anyway, the bottom line is that it's a configuration error
for the destination host of the message.  There's nothing you can do
about it except write to the address that the error message came from
or delete the user from the list.  If you wait a while, the system
administrators there may notice the error and fix it without your
doing anything.