You can use Email marketing to Promote a business directly to the customers. The email is drafted in a way that it includes all the details related to the Products or services. It is highly targeted as you can directly reach to the client and this technique is self measurable. You can easily track the success of your campaign.
You cannot just sent email with your product to a potential customer. First you need to send email to ask if he is interested, then you can send him an offer. Otherwise it is spam.