You really haven't given us much information to work with, but the best resource for this kind of problem is the administrator of the system that is producing the "your message is spam" declarations.  Are the messages being returned by the LISTSERV site, or by a mailserver being used by a subscriber of the owned list?  Can you show an actual spam-reject event in a log file?

As for reputation, usually "an ISP" by name doesn't have one, but the IP address of their outgoing mailserver might.  We can't readily determine that from what you submitted, but it would appear in the headers of one of the messages in question.

>>> Randy Klumph <[log in to unmask]> 06/22/09 2:25 PM >>>
I have a list owner whose messages are being returned as spam. The messages do not seem to have any spam characteristics. Could it have something to do with the reputation of her ISP (@tpg.com.au).

Listserv 15.5
Win 2k
no anti spam software running

Regards,
Randy Klumph
[log in to unmask] 
The National Consortium on Deaf-Blindness
The Teaching Research Institute
345 N Monmouth Ave
Monmouth, OR 97361
http://nationaldb.org