Something that we often recommend is to create a separate email account for just your online purchases and promotions. If the user would forward things to this

Something that we often recommend is to create a separate email account for just your online purchases and promotions. If the user would forward things to this account (either manually or automatically) that would work, but few, if any, places make a distinction between your ability to login and the identity you use to receive email, so you couldn't divorce the authentication from the notification. This account would also get password recovery emails, for example, and potentially expose more than just the receipt emails. It would partition the more personal stuff, but is only a partial solution.