What common mistake is the user making in the PHP code provided for sending emails?

The common mistake in the provided PHP code for sending emails is that the "mail" function is being used without setting the necessary headers, such as the "From" header. This can result in emails being marked as spam or not being delivered at all. To fix this issue, you should set the "From" header using the "additional_headers" parameter in the mail function.

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

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