Check the domains in the email : http://www.barracudacentral.org/lookups/lookup-reputation

If they are listed here you need to get the customer to remove their domain from this by following the instructions on the page

You can also use the forum it seems : https://community.barracudanetworks.com/forum/index.php?/topic/20584-zerohour-intent-catches/

If you host your own barracuda you can whitelist the customer’s domain temporary via this

Go to Basic, Spam Checking

Add the customer domain to : URI Exemptions: