Applies to all Email Security Gateways, all firmware versions.
The sender will receive an error code during the smtp transaction stating the message exceeds the maximum size. The message looks like this:
552 Error: message too large
It will also appear on the Basic > Message Log page as a Mail Protocol Violation. The Email Security Gateway will not send an NDR or bounce message in this case (though the sending mail server may). Barracuda Networks defines "backscatter" as bounceback messages invoked by the spam-scanning engine. In this scenario, it would be the sending server which would notify the original sender of the delivery failure.
The default maximum message size on the Email Security Gateway is 100MB. To change the maximum allowed message size, navigate to the Advanced > Email Protocol page of the Barracuda's web interface and find the Maximum Message Size option. The maximum message size should be entered in bytes (15MB would be entered as 15000000 or, to be more precise, 15728640)
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