validate e-mail address

public static bool IsValidEmailAddress(string sEmail)
{
if (sEmail == null)
{
return false;
}

int nFirstAT = sEmail.IndexOf('@');
int nLastAT = sEmail.LastIndexOf('@');

if ( (nFirstAT > 0) && (nLastAT == nFirstAT) &&
(nFirstAT < (sEmail.Length - 1)) )
{
// address is ok regarding the single @ sign
return (Regex.IsMatch(sEmail, @"(\w+)@(\w+)\.(\w+)"));
}
else
{
return false;
}
}

//--------------------------------------------------------------------------------
public static bool ValidEmailAddress(string addy)
{
    bool valid = true;
    string pattern = @"^[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?$";
    Regex regEx = new Regex(pattern);
    valid = (regEx.IsMatch(addy));
    return valid;
}

using System.Text; 
private bool IsValidEmail(string email)        
{            
    string emailRegEx = @"^[a-z][a-z|0-9|]*([_][a-z|0-9]+)*([.][a-z|"
                      + @"0-9]+([_][a-z|0-9]+)*)?@[a-z][a-z|0-9|]*([-][a-z|0-9]+)*\.([a-z]" 
                      + @"[a-z|0-9]*(\.[a-z][a-z|0-9]*)?)$";
    RegularExpressions.Match match = 
    RegularExpressions.Regex.Match(email, emailRegEx, RegularExpressions.RegexOptions.IgnoreCase);            
    return match.Success;  
}