Uncertified email does not constitute a legal document so I doubt a bank would accept in any case. It's way to easy to spoof email so without proper certification it's not worth the ...... can't say paper but you know what I mean.... that it's written on. As fer as I know, and legal can support, you can use a trail of emails to support a cause but not as legal document themselves.
Maybe a letter emailed as as PDF document? Of course you should have to have the correct email address.