Green application is installed between end user's mail client and POP3 mail server. It reads mail from the server and runs each message received through a list of rules. Some of the rules may be applied to the message and perform certain actions on it. During that scanning, rules can modify the ruleset itself by adding or deleting other rules. Thus the ruleset gets adapted to new mail.

At the end of the scanning, the message is moved to exactly one of the Green internal mailboxes. There are normally two of those - mail (for clean mail) and junk (for spam). Ruleset is stored in an XML file and it's syntax is flexible enough to make it do nearly anything - filter on user, domain, keyword, vacation autoresponder, but the main idea was still having ruleset for auto-responding whitelist.

For each unknown sender, a challenge message is composed and sent and ruleset is modified. Challenge message can read something like "hey, if you are real and care to contact me, please reply with message with (random number) in it's subject". Should a valid response come from the same sender, the ruleset is modified again and all further mail gets right into clean mailbox from that moment on.