Good question. In basic terms, the script simply posts everything that is sent to a specified address. So if bounced messages are sent to the list, they'll be posted too.
But a properly configured mailing list should have bounces going to the list owner already (not the list itself) so it shouldn't be a problem.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.