Hopefully "deactivate" simply means preventing the owner of the account from logging in. It is possible to deactivate an email account while storing incoming email and preserving the contents of that person's account - just in a way inaccessible to them. That way, it could simply be temporary - especially if appealed.

It's an absolutely terrible decision, but I think (hope?) it will be more akin to them changing the lock on your PO Box and not allowing you to access its contents until the situation is cleared up. Clearly not an acceptable decision, but it's better than permanently losing everything and can be appealed by the account's holder.

Plus, if the deactivation was a destruction of the person's data, that would put the bank at huge liability I would have to think. People have important emails and I don't think any jury would side with the bank. Imagine presenting that case to a jury. "The bank screwed up and to cover their ass, they destroyed all my email - all my personal and professional communications; important documents. . ."