And we're set!

front/form.js

 
 var server  = getServer();
 var xhrSend = new XMLHttpRequest();
-xhrSend.onreadystatechange = function() {
-    if (xhrSend.readyState == XMLHttpRequest.DONE) {
-        console.log(xhrSend.response);
-    }
-};
 
 
 // Returns the server's base URI based on the user's script tag
     // Set the form's behaviour
     el.setAttribute('onsubmit', 'sendForm(); return false;');
     
+    // Add an empty paragraph for status
+    var status = document.createElement('p');
+    status.setAttribute('id', 'form_status');
+    el.appendChild(status);
+    
     var input = {
         name: getField(items.name, 'Your name', false, 'input'), // TODO: configurable prefix
         addr: getField(items.addr, 'Your e-mail address', true, 'input'),
     // Adding submit button
     
     el.appendChild(getSubmitButton('form_subm', 'Send the mail'));
+    
+    // Setting the XHR callback
+    
+    xhrSend.onreadystatechange = function() {
+        if(xhrSend.readyState == XMLHttpRequest.DONE) {
+            let status = document.getElementById('form_status');
+            status.setAttribute('class', '');
+            if(xhrSend.status === 200) {
+                cleanForm();
+                status.setAttribute('class', 'success');
+                status.innerHTML = 'Your message has been sent.';
+            } else {
+                status.setAttribute('class', 'failure');
+                status.innerHTML = 'An error happened while sending your message, please retry later.';
+            }
+        }
+    };
 }
 
 
 // Send form data through the XHR object
 // return: nothing
 function sendForm() {
+    // Clear status
+    let status = document.getElementById('form_status');
+    status.setAttribute('class', 'sending');
+    status.innerHTML = 'Sending the e-mail';
+    
     xhrSend.open('POST', server + '/send');
     xhrSend.setRequestHeader('Content-Type', 'application/json');
     xhrSend.send(JSON.stringify(getFormData()));
         subj: document.getElementById(items.subj + '_input').value,
         text: document.getElementById(items.text + '_textarea').value
     }
+}
+
+
+// Empties the form fields
+// return: nothing
+function cleanForm() {
+    document.getElementById(items.name + '_input').value = '';
+    document.getElementById(items.addr + '_input').value = '';
+    document.getElementById(items.subj + '_input').value = '';
+    document.getElementById(items.text + '_textarea').value = '';
 }

server.js

     date: true
 });
 
+
 // nodemailer initial configuration
 var transporter = nodemailer.createTransport(settings.mailserver);
 
+
 // Serve static (JS + HTML) files
 app.use(express.static('front'));
 // Body parsing
 
 // A request on /send with user input = mail to be sent
 app.post('/send', function(req, res, next) {
+    // Count the failures
+    let status = {
+        failed: 0,
+        total: settings.recipients.length
+    };
+    
     // params will be used as:
     // - values for html generation from the pug template
     // - parameters for sending the mail(s)
         logStatus(infos);
     }, function() {
         res.header('Access-Control-Allow-Origin', '*');
-        res.status(200).send();
+        if(status.failed === status.total) {
+            res.status(500).send()
+        } else {
+            res.status(200).send();
+        }
     })
 });
 
 var port = process.env.PORT || 1970;
 // Start the server
 app.listen(port, function() {
-    log.info("Server started on port " + port);
+    log.info('Server started on port ' + port);
 });
 
 
         log.info('Message sent to ' + infos.accepted[0]);
     }
     if(infos.rejected.length !== 0) {
+        status.failed++;
         log.info('Message failed to send to ' + infos.rejected[0]);
     }
 }