[Mailman-Users] Digest members of one domain marked bouncing

Recently we've had a lot of members from one particular domain ( bigpond.com/bigpond.com.au) marked bouncing, and it looks like it's only digest members who are affected. I haven't checked yet if all digest members from that domain are affected. We unticked their nomail box, but a few days later they're marked bouncing again.

I'm guessing that for some reason, that mail server thinks our digest emails are spam. Any ideas what to do about this?

Peter Shute