I think it's a matter of trust too. Most sites you just don't trust with your primary email. If you let the user change it later I see no problem with letting them sign up with a throwaway email, and if you try this hard to stop them, you're likely to just drive users away or drive them to find a service which isn't on your list of banned domains. The painful truth for most sites on the internet is they are just not important enough to warrant giving them access to my inbox.