Explanation:

In online registration its always a tedious mechanism to determin that if enterd email was correct or not.
Here we are talking about  to different processes.

1. Email Validation
2. Email Verification

For Email Validation we can use the expression Validator. Select Email type in the Expression type in the Expression Validator ASP .NET 2.0 Control. This will match the enterd Email with an Expression.
We can also create our own Validator by using Javascript(for cleint side validation) or C#(for server side validation) and Regex Class.

public static bool IsEmail(string Email)
    {
        string strRegex = @"^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}" +
            @"\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\" +
            @".)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$";
        Regex re = new Regex(strRegex);
        if (re.IsMatch(Email))
            return (true);
        else
            return (false);
    } 

For Email Verification we can use three different methods:

1. SMTP Network connection and Handshake
2. MX records Cheking
3. Sending an automatically generated code/Password

string[] host = (address.Split('@'));
string hostname = host[1];
    
IPHostEntry IPhst = Dns.Resolve(hostname);
IPEndPoint endPt = new IPEndPoint(IPhst.AddressList[0], 25);
Socket s= new Socket(endPt.AddressFamily,
        SocketType.Stream,ProtocolType.Tcp);
s.Connect(endPt);

Now, when we are sure that the domain name exist, we can do handshake with the Mail server and see if the email exist or not.

private enum SMTPResponse: int
{
CONNECT_SUCCESS = 220,
GENERIC_SUCCESS = 250,
DATA_SUCCESS = 354,
QUIT_SUCCESS = 221
}
private bool CanHandshake(){
if(!Check_Response(s, SMTPResponse.CONNECT_SUCCESS))
{
s.Close();
return false;
}

Senddata(s, string.Format("HELO {0}\r\n", Dns.GetHostName() ));
if(!Check_Response(s, SMTPResponse.GENERIC_SUCCESS))
{
s.Close();
return false;
}

Senddata(s, string.Format("MAIL From: {0}\r\n", message.From ));
if(!Check_Response(s, SMTPResponse.GENERIC_SUCCESS))
{
s.Close();
return false;
}

Check the MX Records
There is a better way (and also expensive) to do Email verification.We can use ASPMX tool for this activity. This tool checks the MX Records in distanation Email server.
I my self still could not figure out how can we do this through .NET platform. If some one knows then please write in the comments.
Of course, not every body can afford this Tool to buy as the Licence is a bit expensive. This is why we have to see some other way.

Sending a Verification code or an automatically generated password
1st way would be which most websites use, is to create and send a verification code to the newly registerd user and only activate the user when he enterd the right code.

2nd way which is I preferd to use, is to create an automatically generated password (instead of verification code) which the user can only use, when he can get in the email in his inbox.
The presedence which I found in the later was that you dont need to creat extra database record system. That is, if you want that the user enter verification code you have to save the code in a Table which will later let the system match both codes. The disadvantage code be that if the user enterd wrong email he wont be able to access the Account any more and you will get an inactive user in the users list. To minimize this activity, alert or inform the user that if email is enterd wrong then he/she wont be able to use the account.

In both cases you need a snippet which help you create a piece of code. To create automatic password you can use the Membership object.

The method signature look like this:

public static string GeneratePassword ( int length, int numberOfNonAlphanumericCharacters )

public void CreateUser_OnClick(object sender, EventArgs args)
{
  // Generate a new 12-character password with 1 non-alphanumeric character.
  string password = Membership.GeneratePassword(12, 1);

  try
  {
    // Create new user.

    MembershipUser newUser = Membership.CreateUser(UsernameTextbox.Text, password,
                                                   EmailTextbox.Text);

    Msg.Text = "User " + Server.HtmlEncode(UsernameTextbox.Text) + " created. " +
               "Your temporary password is " + password + ".";
  }
  catch (MembershipCreateUserException e)
  {
    Msg.Text = GetErrorMessage(e.StatusCode);
  }
  catch (HttpException e)
  {
    Msg.Text = e.Message;
  }
}
public string GetErrorMessage(MembershipCreateStatus status)
{
   switch (status)
   {
      case MembershipCreateStatus.DuplicateUserName:
        return "Username already exists. Please enter a different user name.";

      case MembershipCreateStatus.DuplicateEmail:
        return "A username for that e-mail address already exists. Please enter a different e-mail address.";

      case MembershipCreateStatus.InvalidPassword:
        return "The password provided is invalid. Please enter a valid password value.";

      case MembershipCreateStatus.InvalidEmail:
        return "The e-mail address provided is invalid. Please check the value and try again.";

      case MembershipCreateStatus.InvalidAnswer:
        return "The password retrieval answer provided is invalid. Please check the value and try again.";

      case MembershipCreateStatus.InvalidQuestion:
        return "The password retrieval question provided is invalid. Please check the value and try again.";

      caseMembershipCreateStatus.ProviderError:
       return"The authentication provider returned an error. Please verify your entry and try again.";

      case MembershipCreateStatus.UserRejected:
        return"The user creation request has been canceled. Please verify your entry and try again.";

      default:
       return "An unknown error occurred. Please verify your entry and try again.";
   }
}

private string GenerateFriendlyPassword(int intPasswordLength)
{
// add any more characters that you wish!
string strChars = "abcdefghijklmnopqrstuvwxyz1234567890";
Random r = new Random();
string strNewPassword = string.Empty;
for (int i = 0; i < intPasswordLength; i++)
{
int intRandom = r.Next(0, strChars.Length);
strNewPassword += strChars[intRandom];
}
return strNewPassword;
}