Anti-spam appliances are hardware

Computer hardware

Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...

 devices integrated with on-board software that implement anti-spam techniques (e-mail) and/or anti-spam for instant messaging

Instant messaging manager

Companies that provide instant messaging management products and services include Akonix, Barracuda Networks, CSC, Global Relay, FaceTime Communications, Presensoft, ScanSafe, Smarsh, SurfControl and Symantec....

 (also called "spim") and are deployed at the gateway or in front of the mail server. They are normally driven by an operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

 optimized for spam

E-mail spam

Email spam, also known as junk email or unsolicited bulk email , is a subset of spam that involves nearly identical messages sent to numerous recipients by email. Definitions of spam usually include the aspects that email is unsolicited and sent in bulk. One subset of UBE is UCE...

 filtering. They are generally used in larger networks such as companies and corporations, ISPs, universities, etc.

Reasons anti-spam appliances might be selected instead of software could include:

• The customer prefers to buy hardware rather than software
• Ease of installation
• Operating system requirements (e.g. company policy requires Linux, but software is not available under this OS)
• Independence of existing hardware