PHP email sent successfully but not received by Gmail - php

Question

I have some code that has worked for me in the past, but isn't anymore, for some reason I don't understand. I'm pretty sure the problem isn't in the code itself, since 1) I haven't changed anything, and 2) it's worked for me in the past. I am using XAMPP and PHP.
My webpage allows a user to fill out a contact form and send an email through PHP. It seems to execute successfully, since it takes the user to "thankyou.html" as a result. To troubleshoot the issue, I have 1) cleared my browser's cache and 2) checked my spam folder in Gmail.
But I still can't find my email, and don't know why. I'm not familiar with how SMTP works; if the problem has to do with that, then I would appreciate an explanation.
Here is the code:
<?php
$field_first_name = $_POST['cf_first_name'];
$field_last_name = $_POST['cf_last_name'];
$field_email = $_POST['cf_email'];
$field_message = $_POST['cf_message'];
$field_reference = $_POST['cf_reference'];
$mail_to = 'myself#gmail.com';
$subject = 'Message from a website visitor '.$field_first_name.' '.$field_last_name;
$body_message = 'From: '.$field_first_name."\n";
$body_message .= 'E-mail: '.$field_email."\n";
$body_message .= 'Message: '.$field_message."\n";
$body_message .= 'Reference: '.$field_reference;
$headers = 'From: '.$field_email."\r\n";
$headers .= 'Reply-To: '.$field_email."\r\n";
$mail_status = mail($mail_to, $subject, $body_message, $headers);
if ($mail_status) { ?>
<script language="javascript" type="text/javascript">
//alert('Thank you for the message. We will contact you shortly.');
window.location = 'thankyou.html';
</script>
<?php
}
else { ?>
<script language="javascript" type="text/javascript">
alert('Message failed. Please, send an email to someemail#gmail.com');
window.location = 'home.php';
</script>
<?php
}
?>
Any input is much appreciated!

Answer 1

At least AOL automatically drops all emails sent by PHP's internal mail (I don't know how they detect it, it simply will not show up in the inbox).
A solution is to use a email library like PHPMailer and send your emails via a real SMTP server.
Edit: Does your local machine have a dynamic or fixed IP address? Almost every serious SMTP server does reject emails sent by dynamic IP addresses.

Answer 2

I am using this: (in my website and not in XAMPP)
$from = "mail#mail.de";
$to = "mail#mail.de";
$subject = "subject";
$mailtext = "blablabla";
mail($to, $subject, $mailtext, "From: $from ");
And it's working!
But I tried it with XAMPP as well and nothing happens :(
So I don't know how to use mail() with XAMPP.

Answer 3

As of Nov 2022
Anyone sending emails to Gmail accounts, including personal accounts ending in #gmail.com or googlemail.com, and Google Workspace accounts for work or school must set up either SPF or DKIM to avoid Gmail blocking or marking your emails as spam.
https://support.google.com/a/answer/33786