msg = new MailMessage("accounts@greektools.net", email);
        msg.Subject = "GreekTools Registration";
        msg.Body =
            "You have been invited by your organization to register for the GreekTools recruitment application.<br/><br/>" +
            url + "<br/><br/>" +
            "Sincerely,<br/>" +
            "The GreekTools Team";

        msg.IsBodyHtml = true;

        client = new SmtpClient();
        client.Host = "relay-hosting.secureserver.net";

        client.Send(msg);

<system.net>
<mailSettings>
  <smtp from="accounts@greektools.net">
    <network host="relay-hosting.secureserver.net" port="25" userName="********" password="*********" />
  </smtp>
</mailSettings>

public class Email
{
    public enum MailAddressType
    {
        From = 1,
        Bcc
    }

    private static MailAddress _from = null;

    public static void SendEmail(string to, string subject, string body)
    {
        SendEmail(to, subject, body, From, string.Empty);
    }

    public static void SendEmail(string to, string subject, string body, string from)
    {
        SendEmail(to, subject, body, from, MailAddressType.From);
    }

    public static void SendEmail(string to, string subject, string body, string addresses, MailAddressType addressType)
    {
        MailAddress from = From;
        string bcc = string.Empty;

        if (MailAddressType.From == addressType)
        {
            from = new MailAddress(addresses);
        }
        else
        {
            bcc = addresses;
        }

        SendEmail(to, subject, body, from, bcc);
    }

    private static void SendEmail(string to, string subject, string body, MailAddress from, string bcc)
    {
        MailMessage message = new MailMessage();
        message.From = From;
        message.To.Add(to);
        if (!string.IsNullOrEmpty(bcc))
        {
            message.Bcc.Add(bcc);
        }
        message.ReplyTo = from;
        message.Subject = subject;
        message.Body = HttpContext.Current.Server.HtmlEncode(body);
        SmtpClient smtp = new SmtpClient();
        smtp.Send(message);
    }

    public static MailAddress From
    {
        get
        {
            if (null == _from)
            {
                SmtpSection section = (SmtpSection)ConfigurationManager.GetSection("system.net/mailSettings/smtp");
                string address = section.From;
                string displayName = ConfigurationManager.AppSettings["fromEmailDisplayName"];
                _from = new MailAddress(address, displayName);
            }
            return _from;
        }
    }
}

<appSettings>
    <add key="fromEmailDisplayName" value="Firstname Lastname"/>
</appSettings>

<system.net>
    <mailSettings>
        <smtp deliveryMethod="Network" from="myname@mydomain.com">
            <network host="relay-hosting.secureserver.net" />
        </smtp>
    </mailSettings>
</system.net>

<system.net>
      <mailSettings>
       <smtp from="abc@xyz.net">
        <network host="relay-hosting.secureserver.net"/>
       </smtp>
      </mailSettings>
    </system.net>

<system.net>
    <mailSettings>
      <smtp from="abc@xyz.net">
        <network host="smtpout.secureserver.net"
           userName="abc@xyz.net" password="your_password_here"
           port="25" />
      </smtp>
    </mailSettings>
  </system.net>

defaultCredentials="false"

<network host="relay-hosting.secureserver.net" port="25" userName="********" password="*********" defaultCredentials="false" />

//call this line, before you call .Send
client.Credentials = CredentialCache.DefaultNetworkCredentials;
client.Send(msg)