[account42]

You could do soft validation that provides protection for common mistakes while still allowing users to use domains you didn't expect.

"Did you mean [e-mail] instead of layer8@gmailc0m [Y][N]".

[layer8]

Indeed, “do you really mean that?” would be useful, though I would always have the user type the correction themselves, because too many users would select “Yes” without thinking or attentively verifying.