How can different email providers like Gmail, Hotmail, and GMX handle emails sent using PHP's mail() function differently?

Different email providers may have different spam filters and security measures in place that can affect how emails sent using PHP's mail() function are delivered. To ensure better deliverability, it is recommended to set proper headers in the email message, such as From, Reply-To, and MIME type. Additionally, using a dedicated email service provider like SendGrid or Mailgun can help improve email deliverability and avoid being marked as spam.

$to = "recipient@example.com";
$subject = "Test Email";
$message = "This is a test email.";

$headers = "From: sender@example.com\r\n";
$headers .= "Reply-To: sender@example.com\r\n";
$headers .= "MIME-Version: 1.0\r\n";
$headers .= "Content-type: text/html; charset=iso-8859-1\r\n";

mail($to, $subject, $message, $headers);