[e12e]

The project name escapes me, but there was a python project that generated email addresses with a hmac, and rejected invalid addresses ([e-mail]). I think it came out a little before the rise of grey listing.

[kevincox]

https://blame.email/ is a website that does this. I wrote a Lua checker for rspamd that bypasses the spam filter if the address is "signed". I also have a bookmarklet that generates a signed address and inserts it into the current text field.