C#

        private void txtboxEmailAdd_Validating(object sender, CancelEventArgs e)
        {
            string pattern = @"^[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][a-z|0-9]*(\.[a-z][a-z|0-9]*)?)$";
            System.Text.RegularExpressions.Match match = 
                Regex.Match(txtboxEmailAdd.Text.Trim(), pattern, RegexOptions.IgnoreCase);
            if (match.Success || txtboxEmailAdd.Text == "")
                return;
            else
                MessageBox.Show("Invalid Email Address" + "\r\n" + "Please enter Valid Email", "Invalid Email Error",MessageBoxButtons.OK, MessageBoxIcon.Error);
            e.Cancel = false;
        }

string s = "Hello@email.com";
if (s.IndexOf("@") <= 0)
   {
   MessageBox.Show("Not an EMAIL address");
   }

Kwagga wrote:

Even if I have to say so myself....

(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21\x23-\x5b\x5d-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21-\x5a\x53-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])+)\])