How can improper handling of email headers in PHP lead to emails being rejected by intermediate systems or not being sent at all?

Improper handling of email headers in PHP can lead to emails being rejected by intermediate systems or not being sent at all due to incorrect formatting or missing required headers. To solve this issue, make sure to properly format email headers using the correct syntax and include necessary headers such as From, To, Subject, and MIME version.

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