The gotcha was that all the students and faculty had .seattleu email addresses,so no one received the bounceback messages. One instructor, who was smart enough to enroll himself with his personal gmail address, finally saw that the emails were being bounced.
We resolved this issue a few weeks ago, but I thought I would mention the "gotcha' in case anyone else encounters it. It turns out that courses1.webwork.maa.org was 'banned"(blacklisted) by the SeattleU email server. This was easy enough to get fixed once we realized that this was the case.