As has happened with many other network services, spam-fighting has reached the stage where network appliances are becoming available at an increasingly rapid rate. The handy thing about appliances is that they come preconfigured to do their job, and if you're not a mail or spam expert you don't even have to worry about what operating system anything else is running. On the minus side, they're more limited than a machine that you can select the software for. They're also ten times or more as expensive as just purchasing the software!
With terms such as "spam firewall," companies such as Barracuda Networks, Tumbleweed Communications, and trimMail offer solutions for those companies that don't want to dedicate entire servers to the task of spam fighting, or don't have the IT staff to maintain more complicated software and operating systems issues.
Barracuda Networks offers a family of Barracuda Spam Firewall appliances, differentiated by the number of mailboxes and messages each is designed to handle. These machines offer DNSBL lookups, Bayesian filtering, basic feature recognizers, whitelists and blacklists, as well as dangerous-attachment checks and virus scanning. Suspicious items are quarantined, and can be made accessible to end users or just to administrators. An additional benefit of this device is its ability to limit the rate at which mail is processed, to protect your mail server against denial-of-service attacks and dictionary attacks. However, any single test that succeeds will cause the mail to be flagged as spam.
On the other hand, Tumbleweed Communications offers the Tumbleweed Email Firewall with their Tumbleweed Messaging Management System (MMS)the MMS software is also available separately. The Tumbleweed Email Firewall, like Barracuda's Spam Firewall, provides DNSBL lookups, feature recognizers, whitelists and blacklists, as well as scanning for viruses and dangerous attachment types. It also provides Bayesian filtering, comprehensive scoring, and supports flexible quarantining options. As an actual firewall, however, this device is hardened against dictionary attacks, denial-of-service attacks, unauthorized attempts to relay mail, and various SMTP exploits that could otherwise affect your mail server.
trimMail offers the trimMail Inbox, and also has quite a number of tips and tricks for email issues on their site, along with various web sites they recommend keeping an eye on for the latest in spam news and spam-fighting techniques. The trimMail Inbox is another full-featured anti-spam appliance, complete with DNSBL lookups, feature recognizers, Bayesian filtering, whitelists and blacklists, dangerous-attachment checks, as well as basic protection against denial-of-service attacks and relaying attacks. There's no quarantine support, though the device can label spam in such a way that the recipient's email client can be configured to filter it into a junk mail folder.
There aren't many more appliances available at the time of this writing, but their numbers are growing by leaps and bounds as companies who have "seen the spam light" finally bring their new products to market. If you're interested in this kind of solution, be sure to look around and see what new offerings are available.