The <resource> element is a child of the<message> element. Its content is treated as the name of a file to be incorporated into the SMTP message a resource for the message body. For example, a .css style sheet for an HTML message body could be supplied as a resource.
cid: Specifies the content ID used to refer to the resource in URLs in the message body. For example, if a .css style sheet is the resource, then the cid value might be css-1. In the HTML message body, the following element can be used to refer to the .css style sheet:
<link href="cid:css-1" rel="style sheet" type="text/css">