Send Mail Header as auth_username insteall of declared variable

Question

Hi guys just wondering why my code display header as auth_username instead of declared variable

function sanitize_my_email($field) {
    $field = filter_var($field, FILTER_SANITIZE_EMAIL);
    if (filter_var($field, FILTER_VALIDATE_EMAIL)) {
        return true;
    } else {
        return false;
    }
}

$to_email = 'la.gelo.fernandez@gmail.com';
$from = 'angelo.roccatech@gmail.com';

$subject = 'Testing PHP Mail';
$message = 'This mail is sent using the PHP mail ';
$headers = "" .
           "Reply-To:" . $from . "\r\n" .
           "From:" . $from . "\r\n" .
           "X-Mailer: PHP/" . phpversion();
$headers .= 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n"; 

//check if the email address is invalid $secure_check
$secure_check = sanitize_my_email($to_email);
if ($secure_check == false) {
    echo "Invalid input";
} else { //send email 
    mail($to_email, $subject, $message, $headers);
    echo "This email is sent using PHP Mail";
}

here's my output

from:   la.gelo.fernandez@gmail.com // this one is my auth_username instead of angelo.roccatech@gmail.com
reply-to:   angelo.roccatech@gmail.com
to: la.gelo.fernandez@gmail.com
date:   Mar 9, 2021, 12:56 PM
subject:    Testing PHP Mail