[inimino]

I wrote about another technique at the end of my own captcha rant: https://inimino.org/~inimino/blog/kill_captcha

The proposed solution would replace captcha entirely, but to my knowledge nobody has tried it.

[basicplus2]

How about different revocable secret keys to your mailbox given only to those who you wish to contact you

[inimino]

If people have to request to be able to contact you, it is not a public inbox anymore. This is the way most walled gardens work, you have a separate step before you can interact, so it works, but it lacks some of the affordances of the public inbox model of email or blog posts, such as allowing anonymity.