Mailer-Daemon as spam sender?

Running Tiger Server 10.4.7. I have five users enabled for e-mail accounts. They are all outside my network. IOW, they use their own ISP's SMPT to send. SMTP is limited to the local network and is password-authenticated. But I'm seeing a 20-30 messages a day -- any is too much -- passing through the system. In the queue, the sender shows as Mailer-Daemon. Lists are disabled. When I check the SMTP log, I see the sender showing as "" yet Postfix accepts it for delivery. One user was showing an unusually high number in quota used, so she changed her password to a healthy mix of numbers and letters. But the nonsense continues. I thought I had done everything required to prevent unauthorized relaying. It would certainly be embarrassing to have my mail server blacklisted as an open relay. Any suggestions here on where I should be looking to plug this hole?