I post to a group I'm subscribed to; messages get spam-binned. Why, if I'm subscribed? And how can I stop it?

Sometimes it happens, sometimes it doesn't: I post to a group, and the message never appears. I ask the moderator to check for me, and sure enough, my message is sitting in the spam-bin, waiting to be approved or denied. (I'm not the only user on this group this happens to, either.)