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You can send an email to all members of your SharePoint site using the following code:

SPSite mySite;
SPWeb currentWeb;

mySite = new SPSite(Url); //Url contains site URL
currentWeb = mySite.OpenWeb();

string sMemberName = "";
string strSubject = "";
string strBody = "";

for (int i=0;i<currentWeb.Users.Count;i++)
{

 sMemberName = currentWeb.Users[i].LoginName.ToString();
 strSubject = "Testing";
 strBody = "This is a test.";

 SendMail(currentWeb,currentWeb.Users[sMemberName].ToString() , strSubject, strBody);
}

//SendMail Function
public void SendMail(SPWeb currentWeb, string _user,string strSubject, string strBody)
{
 
 MailMessage msg;
 try
 {
  SmtpMail.SmtpServer = "smtp.yourserver.com";
  msg = new MailMessage();
  msg.BodyFormat = MailFormat.Html;

  msg.From =  currentWeb.Author.Email;
  msg.Subject = strSubject;
  msg.Body = strBody;
  msg.To = currentWeb.Users[_user].Email;
  SmtpMail.Send(msg);
    
 }
 catch (Exception ex)
 {
  //Exception Handling Code ...
 }
}

Do not forget to add Microsoft.System.Web.DLL in references. Add following code at the top of your page:

using Microsoft.System.Web;

You can also use third party email components. <a href="http://www.aspemail.com">aspemail.com</a> is an example. It is an excellent emailing component that you can use in your .NET applications including SharePoint's.