Test: CSS Attachment
In the <body> of the HTML part, place a tag as follows: <link rel="stylesheet" type="text/css" href="cid:ATTACHMENT_ID"> Then attach a multipart/related text/css file to the email with the MIME header: Content-ID:<ATTACHMENT_ID> Inside that specially crafted text/css file, add a CSS style which performs a http request. Specifically: background-image:url('http://TRACKING_URL/');