No one's mentioned getting a response so far, but I'm intrigued enough to give it a try. I've sent in a question about genetic engineering.
Some notes about the process:
1. 21.co's signup form mandates an image, but the tool they use for file uploads breaks on PNGs (!) silently without any visible warning to the user (!!), you can click on the 'save' button many times with nothing happening and without any kind of error or warning in the web console (!!!), in both Firefox and Chromium with Noscript & Adblock disabled. Apparently even using an entire specialized file upload service, the second-most common image format in existence is just too exotic and confusing to support.
2. you have to confirm by email before you can do anything, which adds on another 10-15m to the process (amusingly, they spam you on signup to set up a public paid inbox - and that email arrived first, but you can't do anything unconfirmed)
3. it's not $100, it's actually $110, because 21co tacks on a $10 'service fee' (even though it's for charity). It's unclear if I have to pay this $10 regardless of whether Andreessen ever responds. I hope not. In any case this strikes me as a huge fee for such a trivial service, and I really hope it's a 10% fee rather than a fixed $10 fee...
4. the site is surprisingly slow despite being so barebones
5. browsing the interface, despite the prominence of Bitcoin, I'm not clear whether I am even allowed to deposit Bitcoin and pay that way; in any case I opted to use a credit card because the profile image bug wasted so much of my time and I was losing my patience. The cash out page indicates that should I ever earn anything via 21.co, I would need to jump through even more hoops, presumably even if I only wanted Bitcoin and not bank deposits or anything. I thought briefly about setting up a paid inbox because it's a fun concept but between a 10% (or worse) fee and all this invasiveness, I'm not interested.
6. signup form doesn't work with Lastpass, which failed to capture the username/password; always annoying to manually copy over generated passwords.
So, I'm not impressed but we'll see how it works out...
On a side note, how do people regard the expectation of privacy with these public inbox emails? I see someone has screenshotted a Horowitz response; are these responses considered 'commissioned' in a sense, rather than private emails with an expectation of privacy/confidentiality, and so it's OK to copy-paste any response publicly on HN or elsewhere?