ValidaEmailAntigo.cs

var listaDeEmails = new ListaDeEmails().Buscar();

var emailRegex = @"^(?!\.)(""([^""\r\\]|\\[""\r\\])*""|([-a-z0-9!#$%&'*+/=?^_`{|}~]|(?<!\.)\.)*)(?<!\.)@[a-z0-9][\w\.-]*[a-z0-9]\.[a-z][a-z\.]*[a-z]$";

foreach(var email in listaDeEmails)
{
    var regex = new Regex(emailRegex, RegexOptions.IgnoreCase);
    var oEmailEstaValido = regex.IsMatch(email);
    if (oEmailEstaValido)
    {
        //Alguma operacão aqui...
    }
}