Email has become not just a useful convenience, but a preferred method of communications for millions of people. It stands to reason that marketing to email addresses would be a useful tool, especially because it is far cheaper to send emails than it is to send postal mail. However, far less reputable businesses have learned this lesson as well, and we all know about the hundreds of annoying "spam" messages we receive in our boxes each day.
Especially when you are looking to establish a relationship with a client, email lists are useful as long as you have the recipient's permission to receive such a message. Also, don't expect one particular message to yield tremendous results. It is usually necessary to "drip on" someone for a period of time so that you eventually come to mind when they need your product or service. If you can afford the time and patience that are needed to market in this way, you can be successful.
I recommend two mail services, both of which I have used, and neither of which I am currently using (I am building an email list): Constant Contact and Mailchimp.