I've done a bunch of experimenting and I now agree with what you told me (sorry about that). I don't know why I thought that the new postings were showing up in the subscribed topics list ... perhaps that occurred while we were trying to force the system to send emails. (My apologies for incorrect information.)
It appears that the lost email problem is happening because the subscriber's system was rejecting it. I updated the reply email address in the forum email configuration setup to be my own, and I started getting bounces that I had previously missed. Now I have something I can use to solve the problem. Thanx.