HTML mail leads to "lack of content" on Apple mobile devices

I am sending HTML mail via PHP mail (). Content is some div containers and some external images.

On Apple mobile devices, such as the iPhone and iPad, the contents of the mail cannot be read; instead, the message “This message has no content” appears. The funny thing is that in the preview area of ​​the mail application, I see the first few words of the mail. Thus, the content is, at least in some way. Everything is fine when you read the same mail in the Google web client.

You can help?

Edit: as requested, here is the source. $ mail is HTML-mailbody.

    $id = md5(uniqid(time()));
    $header= "From: Mailer <info@domain.com>\n";
    $header.= "Content-Type: multipart/mixed; boundary=".$id."\n";
    $header.= "This is a multi-part message in MIME format\n";
    $header.= "--".$id."\n";
    $header.= "MIME-Version: 1.0\n";
    $header.= "Content-type: text/html; charset=iso-8859-1\n";
    $header.= "Content-Transfer-Encoding: 8bit\n";
    $header.= $mail."\n";
    $header.= "--".$id."--";

    $betreff = mb_encode_mimeheader("subject", "UTF-8");

    mail($mailaddress, $betreff, $mail, $header);