Commercial e-mail messages often contain images, graphics and other design
elements. When a message is sent in the HTML format, images used in the message
body can be either linked or embedded.
Linked images are not part of the message itself. They are usually hosted by the message
sender and referenced in the message body via an <IMG> tag pointing
to the sender's server, as follows:
<IMG SRC="http://www.sender-company.com/images/flower.jpg">
The main advantage of linked images is that the message body remains small. The main disadvantages
are that such messages cannot be viewed off-line, and they have a limited
lifespan (images on the sender's server may be moved, deleted, or the server itself
may go offline).
Unlike linked images, embedded images are part of the message itself.
They are similar to regular attachments,
although they use the multipart/related format,
as opposed to multipart/mixed. An embedded image
is also referenced in a message body using an <IMG> tag,
but instead of a URL, an image identifier, or Content ID, is used, as follows:
<IMG SRC="cid:My-Image-Identifier">
A message with embedded images can be viewed off-line and remains
intact over time, as images are permanently attached to the body.
A message may contain regular attachments and embedded images at the same time.
