PostArmor

PostArmor is an application that helps to keep your electronic mailbox free of unsolicited e-mail, better known as spam: it looks at the relevant parts of your email before it leaves the mail server, and lets the legitimate messages pass through undisturbed, while leaving suspect messages on the server to be reexamined later or just deleted after a while.

It works as an email proxy, with support for POP and IMAP, that makes it independent from the e-mail client. As it has been developed in Java, it is also independent from the operating system or platform.