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?