I followed the instructions from
RestrictPostfixRecipients - Zimbra :: Wiki verbatim. The system worked after I did it the first time. The error started after I added additional valid senders. I just tried the restriction configuration again from scratch and it didn't work.
Any thoughts? Are there limits on the number of valid senders the system can hold? That doesn't seem likely, but I'm not ordained in Postfix Arcana, so I don't know for sure.