h a l f b a k e r y
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An email client (or server), that would, upon receiving an
email from a previously unrecognized source, send an email
back with a captcha style problem to be solved by the email's
originator prior to making the email visible to a human
Mailwasher is not in any way associated with or afilliated to or Assimilated by BorgCo. [8th of 7, Jan 27 2020]
||Mailwasher <link> does something a bit like this ... it allows you to preview your mail and bounce it if you don't like the preview.
||The bounce message can include some human-readable instructions like "Add the phrase 'Happy Frost Biscuit' to the subject line to permit delivery'" . Then if it reappears with the nonsense phrase, you know it's genuine.
||This sounds like something, like an autoresponder, that each of us could
run, by using Gmail API to change message flags, and a verification
endpoint hosted somewhere, even on our own websites. [+]