Email validation util.

validateEmails.js

// It splits by comma then map and trim each emai, then check each email pass the test, keeping the ones that do not pass the test.
// If any email do not pass the regex test show it to the user, otherwise continue (return)

const re = /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;

export default emails => {
  const invalidEmails = emails
    .split(',')
    .map(email => email.trim())
    .filter(email => re.test(email) === false);

  if (invalidEmails.length) {
    return `These emails are invalid: ${invalidEmails}`;
  }

  return;
};

Usage example with React-Form:
errors.emails = validateEmails(values.emails || '');