Interac e-Transfers (the only widely used method for doing this I'm aware of) do let you send money to someone via an email address, but it's a notification channel, nothing more. An account enumeration isn't possible with it, the actual email is sent some time after the money for the transfer has left the source account, and the sender doesn't get any information about the delivery of the email.
I suppose you could send e-Transfers to random email addresses and then see if any are accepted, but that would cost you an absolute minimum of $0.01 per attempt and would probably have a terrible response rate.
Source: this is my day job.
Edit: sorry, forgot to mention this is Canada-specific.
