Ask HN: Service that blocks new inbound emails from unapproved senders

[shartshooter]

I had seen a service in the past where, if you're emailing me for the first time, it asks you to do one of two things:

1) verify that you know me by answering a simple question

2) make a donation to a cause I care about so your email goes through

I'm pretty diligent about unsubscribing from all the crummy business newsletters I somehow get signed up for, but am looking for something a bit more aggressive.

In short: I'm looking for a service to keep out all emails except for the few senders I care about

[LinuxBender]

The simplest thing I know of would be to set up a couple Postfix VM's and have rules to allow relaying specific domains to a final mail server of your choice and either discard the rest if confidence is high or put them in a quarantine queue or text file for manual review, or just reject anything not specifically allowed. This allows adding other anti-spam mitigations before anything even touches your final destination mail server.