Valid emails being deferred through Barracuda 100 Spam/Virus Firewall

We have a small business network with a Barracuda Spam Virus Firewall and over the past few months I have seen an increase in the number of deferred messages for valid email addresses.  It's not all emails for certain domains, in fact for one domain I have seen some come right through and others never make it.

Talked to Barracuda support, upgraded to the latest firmware, increased timeout, etc and they say it must be upstream.  I did some further testing by taking the firewall out of the mix while watching some specific deferred messages, and those messages dropped right through to Exchange, so I'm not convinced it's an upstream issue?

My configuration is Qwest DSL with fixed IP and have port 25 forwarded to the Barracuda on the DSL modem. Don't have the firewall enabled on the modem, just NAT.

Read some other articles, but no solutions.  Any ideas what to check next?
KZimAsked:
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SommerblinkCommented:
Well, technically it is an upstream issue.. its just not that far away upstream.

Things to look for are the number of sessions that your DSL/NAT/PAT device can handle. Whether your DSL/NAT/PAT device can handle concurrency to the same inbound port.

I would suggest purchasing a proper firewall verses relying on something free... especially since you've already taken several steps in the right direction towards spam elimination, the Barracuda... sorry, I am very partial towards that product line :)

Even something like the Cisco RV series would probly be a large step forward, without a large capital investment. (http://www.cisco.com/cisco/web/solutions/small_business/products/routers_switches/small_business_routers/index.html)
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KZimAuthor Commented:
Just to make sure I understand your suggestion, it's that the DSL model (an ActionTec M1000) potentially isn't robust enough to handle the incoming traffic appropriately.  So look to replace it with a more robust router (such as the Cisco), right?
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KZimAuthor Commented:
In looking at the Cisco line, these aren't ADSL routers, so my Qwest DSL modem would stay in place, right?  How would that hellp the throughput.  Or, do I need to look at another product to replace the actiontec competely?
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