If you are getting emails bounced back with errors similar to -
----- The following addresses had permanent fatal errors -----
(reason: 554 5.7.1 Service unavailable; Client host [xxx.xxx.xxx.140]
blocked using b.barracudacentral.org;
..go to the URL listed (in *your* bounced email) and it will generally list an error such as "poor". This can be due to the fact that your server has actually sent spam in the last 30 days or a site on the server was compromised and was used to send email until it was noticed and shut down or the IP address of the server doesn't have a high enough mail volume, among other potential "triggers". Commonly, this is due to an overly aggressive Barracuda mail administrator setting additional options in their Barracuda mail appliance causing it to be over-aggressive.
The Barracuda Spam Firewall has rules that apply to email sent from an IP address known to Barracuda Central with a "poor" rating. The Barracuda Spam Firewall has an option to decline email from these IPs. This is an option that the Administrator of the Barracuda Spam Firewall may enable. If the Administrator chooses to enable this option it may block email from your IP address.
Notifying the recipient of this issue is best. They can then contact their mail administrator or change their rules in the Barracuda filter because they would most likely be blocking other emails unintentionally in their attempt at blocking the most spam possible.