Applies to all inbound Barracuda Spam Firewalls not using the Outbound / Relay feature, all firmware versions.
You might want to double check to see if the Barracuda is configured to send bounce messages to senders when a message is blocked for to spam. You can do this by looking in the Barracuda's administration interface on the Advanced -> Bounce/NDR settings -> Spam NDR (Bounce) Configuration . The option is called Send Bounce.
If this value is set to Yes, all bounce messages that the Barracuda cannot deliver will remain in the outbound queue. This often happens when the receiving mail server is unavailable, or the original sending domain does not exist.
Barracuda recommends setting the Send Bounce option to No to optimize performance and minimize backscatter. Regardless, bounce messages stuck in the outbound queue will remove themselves after 48 hours.
For more information on preventing backscatter, see Solution #00001720.
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