You can allow basically anything as an email address (exactly one @ symbol, text ahead and after, with at least one . after, something like /^([^@]+@[^@]+\.[^@]

You can allow basically anything as an email address (exactly one @ symbol, text ahead and after, with at least one . after, something like /^([^@]+@[^@]+\.[^@]+)$/), and still verify that the user is human (captcha or something) and verify the email address (send signup email).