Put a drop-down in the email notification listing all the accounts that are currently associated. The default selection is the account for which the reply was sent; 2nd is the Main Account of the association (oo. and here's where shared things get into some fuckery. I think the answer to that is that there's a subscription for each account, not for the associated secondary user that All Share, so there's no question about who is getting the notif) and then the rest of the account's journals alphabetically.