Spam Arrest uses Challenge/Response technology to keep the spam out. Emails from unknown senders will receive an auto-reply message asking them to verify themselves.
When the sender clicks the link in the email, they are taken to a webpage where they are "challenged" to type in a word that is hidden within a graphic or a sound file (for visually-impaired users).
When the sender successfully completes the "response", their email is forwarded to your inbox, and they are then added to your authorized sender list.
The challenge/response process is easy for people, but impossible for automated systems (i.e. spambots), to complete.
