Barracuda 400 and Exchange 2000 Issue

We installed a Barracuda 400 to act as a smart host for our Exchange 2000 e-mail server.  Both sit behind the firewall (Cisco ASA) on the private network.  There is an intermittent problem when the Barracuda is acting as the smart host for Exchange.  After a short amount of time, when we send mail external to an internet address (verizon.com, comcast.com, aol.com, etc) the senders start receiving NDR's indicating they do not have permission to relay.  After another short amount of time, the e-mails go through with no problem.  The time averages between several seconds and 20 - 30 minutes.    If we change the Exchange server to utilize DNS instead of identifying a smart host, in the Exchange System manager there are no issues.  No NDR's are generated to the users indicating the do not have permission to relay.  The DNS configuration presents it's own problems with ISP reverse name resolution and e-mails are rejected by some ISP's because of reverse DNS lookup issues.  So, the DNS configuration is not a permanent solution and we need to utilize the Barracuda smart host configuration.  We cannot pin down the problem as to if it is a Barracuda issue or an Exchange 2000 server configuration issue.  We have spoken to several Barracuda engineers and Microsoft Exchange engineers and no one has found a solution for this problem.  We are unable to determine if the Barracuda is rejecting the e-mails.  Logs from the Barracuda, Exchange, and Wireshark protocol analyzer has not identified any problems.  Any thoughts, inputs, etc, would be greatly appreciated.