Mail Server: Zimbra 6.0.9
Spam-Firewall: Barracuda Spam Firewall 300 (Firmware v6.1.5.006)
All spam firewalls only work so well, and unfortunately some spam will come through. Incoming spam will often attempt to send messages to invalid addresses on a valid domain. Zimbra reacts by trying to send a Non Delivery Report to the malicious sender (who spoofs an invalid address/domain). Barracuda cannot establish a connection with this spoofed address, and leaves the NDR in the Outbound-Queue in a deferred state.
These failed NDRs are simple to delete from the Outbound-Queue, but can pile up if unattended, on occasion over the limit (100) which email alerts are sent. Can Barracuda automatically delete these messages more quickly than the default 48 hour purge period? It would be helpful if there is a way to do this without disabling NDRs, instead to delete them if they appear to be non-deliverable.
Spam should obviously be mitigated as much as possible to prevent this scenario. But as mentioned, some spam will (and does) always come through.