MailMessage message = new MailMessage();
message.Subject = this.textBoxSubject.Text.Trim();
message.From = new MailAddress(this.textBoxFromMail.Text.Trim());
message.Body = this.richTextBoxContent.Text.Trim();
message.IsBodyHtml = true;

string strConnection = "Server=(local);initial catalog=mail;user id=sa;password=sa;Connect Timeout=30";    
SqlConnection objConnection=new SqlConnection(strConnection);
try
{
    objConnection.Open();
    MessageBox.Show("数据库连接成功！");
}
catch (Exception ee)
{
    MessageBox.Show(ee.ToString());
}

SqlCommand sqlCommand = new SqlCommand("Select * from mail", objConnection);
SqlDataReader reader = sqlCommand.ExecuteReader();
while (reader.Read())
{
    message.To.Add(reader["address"].ToString());
    MessageBox.Show("地址添加成功！");
}
objConnection.Close();

SmtpClient smtpClient = new SmtpClient();
smtpClient.Send(message);
MessageBox.Show("邮件发送成功！");

<configuration>
  <system.net>
    <mailSettings>
      <smtp>
        <network host="smtp.163.com" password="" port="25" userName="" defaultCredentials="false"/>
      </smtp>
    </mailSettings>
  </system.net>
</configuration>