All Barracuda Spam Firewalls.
This message is a result of the setting of your email server rejecting the message for being "over the size limit" of your email server. You are seeing this in the message log because the Barracuda logs the status of messages that your email server has told the Barracuda. Thus a message of this type, of delivery status "552 5.3.4 Message size exceeds fixed maximum message size," is an actual rejection by the email server itself. In this event, it is recommended that you adjust the Barracuda to block the same size as the email server, this way a message that will not be allowed by the mail server will not be allowed by the Barracuda.
If the Barracuda is allowed to reject messages for the same size limit as the email server, the messages will be appropriately stopped at the original inbound message stage, and the sender will be notified appropriately.
If you have Exchange 2007 or 2010 you can follow this link for more information regarding the settings in your Exchange Server.
Part of the "accepted answer" from the TechNet forum can be found here.
And, for Exchange 2003, please follow this link.
Link to this page: