Situation: A prospect's email address has been obtained through some customer touchpoint (e.g., call center, email inquiry, sales call, et cetera), but there has not been any specifically granted permission to email the prospect.
Stance: Nothing has been granted and no business relationship exists, so this recipient is a stranger to you; do not contact this prospect via email.
Solution: Typically, these are out-of-reach people unless you have other contact information. A phone call or direct mail asking them to register for your email promotions or newsletter -- a compelling reason/offer is key -- on a unique landing page is an expensive but suitable option. Very aggressive types may choose to send an email asking these people to opt-in for future email communications.