[layer8]

While I’m opposed to opinionated validation as well, you seem to be missing the issue it tries to solve, which is the user mistyping their email address, not receiving the verification email, and either thinking everything is fine, or thinking that the process is borked, and in any case not proceeding and not becoming your customer. The goal of opinionated validation is to inform the user about an incorrect email address immediately when they are entering it, so they can correct it right away.

[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.