JSFiddle - React, Tailwind, and code Playground

HTML

<p>Insert emails seperated by a comma</p>
<textarea id="emails" cols="80" rows="10" ></textarea>
<br />
<br />
<div id="error"></div>

CSS

#error {color: DarkRed;}

JavaScript

var textarea = $('#emails');

textarea.on({
    keyup: function(e) {
        if (e.which === 188) check();
    },
    blur: check    
});

function check() {
    var val  = $.trim(textarea.val()),
        err  = '';
    
    if (!val.length) {
        err = 'No input ?';
        return;
    }
    
    var emails   = val.split(','),
        notvalid = [],
        temp     = [];
    
    $.each(emails, function(_,mail) {
        mail = $.trim(mail);
        if ( mail.length ) {
            var m = mail.match(/([a-zA-Z0-9._-]+@[a-zA-Z0-9._-]+\.[a-zA-Z0-9._-]+)/gi);
            if (m) {
                temp.push(m);
            }else{
                temp.push(mail);
                notvalid.push(mail)
            }
        }else{
            temp.push(mail);
        }
        if (notvalid.length) err = 'Not valid emails : ' + notvalid.join(', ');
    });
    
    $('#error').html(err);
    textarea.val((temp.length ? temp : emails).join(', '));
}