Stopping it is one of the most crucial jobs a network spam filter tackles:

open relay - An e-mail server that lets third-parties relay e-mail messages. This opens the door for spammers to use your network as a spam factory. All too often, this leads to crashing servers and getting your email blacklisted. That’s bad for business. trimMail Inbox prevents spammers from using your Microsoft Exchange or other e-mail server as an open relay.

Barracuda users have been complaining that an essential feature of Barracuda spam filtering appliance actually turns Barracuda spam appliances into open relays.

Posted February 10, 2005: “If a sender’s domain or the Barracuda’s own domain is whitelisted… all rules are bypassed and the Barracuda becomes an open relay for all e-mail sent from the whitelisted domain.”Although I found this bug last week while evaluating the Barracuda Spam Firewall Model 200 (Firmware 3.1.10), a quick search of Barracuda Networks’ forums revealed other customers had complained about the same problem.

“The initial vendor response was misleading and inferred that the Barracuda will only become an open relay if you whitelist your own domain.”

After some prodding, however, Barracuda apparently finally fessed up:

“Basically anything… that causes the email to be white listed will bypass the scanning engine. This is going to cause the Barracuda to be a open relay…”

Barracuda now claims they’ve fixed this… oversight.

If you’re wondering how any legitimate spam appliance vendor could market an appliance that actually helps spammers spam… Join the club.