How about a signup flow where the user sends the first email? They send an email to [email protected] (or to a generated unique address), and receive a one-time sign-in link in the reply. The service would have to be careful not to process spoofed emails though.
Another approach is to not ask for an email address at all, like here on HN.
Another approach is to not ask for an email address at all, like here on HN.