I know of at least one startup who do this and use a "machine learning algorithm" to find details of particular things in your inbox.

That "algorithm" is a guy in their office. He reads your email. He searches it for common keywords, uses some regexes, but ultimately reads your email, then copies bits out into the system.

I'm all for bootstrapping with things that don't scale, but this example was a bad-faith use of Gmail access. I'm glad I don't use Gmail so couldn't accidentally give a random guy access to my inbox.