React-App Axios POST to php.file with sentMail ->works from localhost but empty $_Post['***'] from Server - reactjs

Question

I have an mail.php file in root on my hosting space.
Do I call mail.php with axios from my localhost from within the development server (npm start) it works fine.
The form who triggers the POST sends the data to my Email address.
After the build process and the upload on my web space, I still receive an email but there the input is missing.
Axios POST:
const formData = new FormData();
// formData.append('name', datas.name);
// formData.append('email', datas.email);
// formData.append('message', datas.message);
// formData.append('nice', nice)
formData.append('name', 'Klara');
formData.append('email', 'klara#gmail.com');
formData.append('message', 'Hallo');
//POST data to the php file
try{
axios.post('https://salevsky.net/mail.php', formData, {
headers: {
'Content-Type': 'multipart/form-data'
}
});
} catch(error) {
PHP file:
<?php
header('Access-Control-Allow-Origin: *');
header('Content-Type: multipart/form-data');
$to = "blablub#webyy.net";
$subject = "Contact Us";
// // Email Template
$message .= "<b>Message : </b>".$_POST['name']."<br>";
$message .= "<b>Email Address : </b>".$_POST['email']."<br>";
$message .= "<b>Message : </b>".$_POST['message']."<br>";
$header = "From:".$_POST['email']." \r\n";
$header .= "MIME-Version: 1.0\r\n";
$header .= "Content-type: text/html\r\n";
$retval = mail ($to,$subject,$message,$header);
// // message Notification
if( $retval == true ) {
echo json_encode(array(
'success'=> true,
'message' => 'Message sent successfully'
));
}else {
echo json_encode(array(
'error'=> true,
'message' => 'Error sending message'
));
}
?>
I tried various DataTypes and ContentForms.
At least headers: { "Content-Type": "multipart/x-www-form-urlencoded" },
I wonder why content type in request header is not changing. I see this in the Browser under Network. It's application/json all time. But I don't know if it's an important fact.