If the episode was released in English first, then the credit should simply include the attribute (voice) - however if the title was first released in another language such as Japanese, the English credit should be listed as (voice: English version).
You can send screenshots of those e-mails via IMDb help desk, which is a more private way to send e.g. proof. And you can blank the e-mail address (and other more sensitive info). IMDb is only interested about info/facts, not your personal messages.