Anti-spam appliances are hardware devices integrated with on-board software that implement anti-spam techniques (e-mail) and deployed at the gateway or in front of the mail server. They are normally driven by an operating system optimized for spam 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