Comodo Anti Spam is a freeware program which protects your email inbox from those nasty spam messages. It does this via a whitelist service which allows messages through from people you trust and permit, and also by running a challenge/response service with any message from a sender it doesn’t recognise.
These challenge response (e.g. please send us the word in the attached image) things are a pain for senders, but useful if you’re seriously under attack by spammers. My anti-spam system is called GMail, but for those without the luxury of this Rottweiller solution, this could be a good option.
If senders of an email are not on the “allowed” or “blocked” lists, the message is temporarily stored, and a special Anti Spam Alert (ASA) message is automatically sent back to the senders. The ASA message includes the Anti Spam Passcode via a graphical attachment. The ASA message asks the sender to reply and type in the passcode in the body of the reply. The idea behind sending the passcode as a graphic is to require a human to read the code and then type it in the reply. An automated actual spammer can’t reply correctly, so their mail gets stored with other junk.