Be more specific on how you build the HTML mail message.
The result will be a multipart-MIME message with a text/html part (if you really do it right with an alternate part of type text/plain) and several images, which are then referenced from within the HTML.
See RFC 1813 and RFC 2378 for more information about content-id in mixed MIME and related data (referred by CID in the HTML source).
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…