Try this:
void EmailValidation(object sender, CancelEventArgs e)
{
System.Text.RegularExpressions.Regex rEMail = new System.Text.RegularExpressions.Regex(@"^[a-zA-Z][\w\.-]{0,68}[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$");
if (txtEmail.Text.Length > 0)
{
if (!rEMail.IsMatch(txtEmail.Text))
{
MessageBox.Show("Invalid e-mail id.", MessageBoxButtons.OK, MessageBoxIcon.Warning);
txtEmail.SelectAll();
e.Cancel = true;
}
}
}