Perfect email config to use on mail() php function

simple-mailer.php

<?php
    
$mail_to = filter_input(INPUT_POST, 'to', FILTER_VALIDATE_EMAIL);
$mail_subject = filter_input(INPUT_POST, 'subject', FILTER_SANITIZE_STRING);
$mail_message = filter_input(INPUT_POST, 'message'); // user can send HTML here
$from_name = 'Your Name';
$from_mail = '[email protected]';

if (!$mail_to || !$mail_subject || !$mail_message) {
    die('Invalid data! Must contain a TO, SUBJECT and a MESSAGE at least');
}

$encoding = "utf-8";
$nl = "\r\n";

$subject_preferences = [
    "input-charset" => $encoding,
    "output-charset" => $encoding,
    "line-length" => 80,
    "line-break-chars" => $nl
];

$boundary = md5(time());
$mail_message = str_replace("\n.", "\n..", $mail_message);

$headers = [];
$headers[] = "Content-type: text/html; charset=" . $encoding;
$headers[] = "From: ".$from_name." <" . $from_mail . ">";
$headers[] = 'To: Mary <[email protected]>, Kelly <[email protected]>';
$headers[] = 'Cc: [email protected]';
$headers[] = 'Bcc: [email protected]';
$headers[] = "MIME-Version: 1.0";
$headers[] = "Content-Transfer-Encoding: 8bit";
$headers[] = "Date: " . date("r (T)");
$headers[] = 'Reply-To: [email protected]';
$headers[] = 'X-Mailer: PHP/' . phpversion();
$headers[] = "Message-Id: <" . uniqid() . "@test.com>";
$headers[] = iconv_mime_encode("Subject", $mail_subject, $subject_preferences);
$headers[] = "Return-Path: [email protected]";
$headers[] = "X-Priority: 2"; // 1 = High, 3 = Medium, 5 = Low
$headers[] = "Importance: Medium";
$headers[] = "X-MSMail-Priority: Medium"; // Compatibilidade com clientes antigos
$headers[] = "User-Agent: Your Mailer Name";
$headers[] = "Content-Type: multipart/alternative; boundary=\"{$boundary}\"";
$headers[] = "X-Tracking-ID: " . uniqid("track_", true);
$headers[] = "Content-Transfer-Encoding: quoted-printable";


$headers_string = implode($nl, $headers);

$mail_message_text = strip_tags($mail_message);

$mail_body = "--{$boundary}{$nl}";
$mail_body .= "Content-Type: text/plain; charset={$encoding}{$nl}";
$mail_body .= "Content-Transfer-Encoding: quoted-printable{$nl}{$nl}";
$mail_body .= quoted_printable_encode($mail_message_text) . "{$nl}{$nl}";

$mail_body .= "--{$boundary}{$nl}";
$mail_body .= "Content-Type: text/html; charset={$encoding}{$nl}";
$mail_body .= "Content-Transfer-Encoding: quoted-printable{$nl}{$nl}";
$mail_body .= quoted_printable_encode($mail_message) . "{$nl}{$nl}";

$mail_body .= "--{$boundary}--";

$result = mail($mail_to, $mail_subject, $mail_body, $headers_string);

if (!$result) {
    error_log("Failed sending email to: $mail_to");
    die("Error trying to sending email to: $mail_to");
}

echo "E-mail sent successfully!";

Use this as you need and update according to your necessities, but give preference to use Symfony/Swift-Mailer which is the best PHP Mail Sender today, also, ensure your domain's DNS have DKIM, DMARC and SPF correctly configured to avoid spam