An important thing to consider is the CAN-SPAM Act, a federal law that sets standards for the sending of electronic mail. The acronym CAN-SPAM comes from the name of the original bill and suggests its goal: Controlling the Assault of Non-Solicited Pornography And Marketing. I'm not a lawyer, but the basics of staying legal include:

• Using a subject line and message that is accurate, relevant and not deceptive.
• Providing an unsubscribe method for future messages (opt-out)
• Including your physical address with every email message
  

Unless you're willing to study the CAN-SPAM Act carefully, a convenient way to help ensure your 'following the rules' is to use an email marketing service. Search the Internet for "email marketing" and you'll find companies that can help you legally send email messages. It is customary to send email messages to persons that requested them (opt-in) only and most email marketing providers will not allow you to send unsolicited messages.