Highlight missing form fields one at a time or all at once?

A form has many required fields some of which have been left blank. When the user clicks the submit button, there is functionality in place to highlight the first required-but-empty field by giving it a red outline and a warning icon beside it. Is this ideal (as it doesn't overwhelm user with warning colors and warning icons all over the form) or would it be better UX to highlight all missing fields at once?