Challenge Response Spam Filter

The challenge-response spam-filter troubles me lately. If you don’t know it, here is how it works if both parties have a challenge-response spam-filter:

1. I write an email to someone@domain
2. The address gets whitelisted on my machine
3. The receiver doesn’t get my message
4. I get a message from the receivers mail-server to which I should reply
5. I reply to the automatic message
6. The receivers mail-server whitelists my address and delivers my initial mail

So far so good. Three questions pop up:

• What if the spammer uses my whitelisted email address to send his spam?
• What if only one of the two has a challenge-response spam-filter?
• What if spammers start to automatically reply to those messages?

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