Barracuda not delivering external email

Well, you have a few odd issues. Given the old firmware, you're subject to a massive bug related to the Spamassassin filter used by the Barracuda. This bug basically bricks all appliances that have a firmware older than January 2012. Anyone with an older firmware is referred to the Barracuda renewals department.

The current firmware is 5.1.1.xxx. Your vitals are fine in terms of CPU usage and disk utilization. The storage on a busy unit plateaus at 75% and logs/etc are rotated to keep things at that level.

You should check to see if you have LDAP integration enabled on your Barracuda.

That should be Domains -> Domain Manager -> yourdomain.com -> Users -> LDAP Configuration

For the time being, change Exchange Accelerator/LDAP Verification: to NO if it's enabled. If you did have it enabled, check your LDAP bind username and password.

If that's not the case, then check the main system configuration.

Basic -> IP Configuration -> Destination Mail Server TCP/IP Configuration.

Change that field to the IP of your Exchange system and input a test address on your network. Run the SMTP test. The output should end with:

From: <[email protected]> 
Date: Wed Apr 4 08:19:55 2012 
Subject: Test message 

This is a test message from your Barracuda Spam & Virus Firewall. 
. 
250 2.6.0 <[email protected]> [InternalId=466] Queued mail for delivery 

Do you have a VMware environment? If so, I would suggest dumping the hardware unit at this point and picking up an evaluation of the Barracuda VX300 virtual appliance. It's far cheaper and faster than the hardware and you can run it for 30 days while "the powers that be" decide on whether they value spam filtering. You won't get support from Barracuda on your existing hardware without paying back to the date that the initial contract ended. It's their policy.

Also see: What to do with an out-of-warranty Barracuda Spam/Web filter?