Random numbers thing is a hack - don't expect it to work reliably
because its up to the configuration for the various proxies and
browsers.

HTTP headers are more than likely the way to go & yes you can set them
in the file if its a .cfm file (using CFHEADER) or on the web server.

The reason I am interested in whether or not its caching in IE is that
IE's caching behaviour  should be easy to control with HTTP, but I
have a sneaking suspicion that Outlook will ignore HTTP stuff and just
cache images as greedily as possible. I think this would be considered
a feature of a mail client rather than a bug - the content of an email
is generally considered static and is not expected to change once the
email has been received.

To get anywhere with this you're going to need to trace what HTTP
headers are getting sent and when (on first load & subsequent loads).

A normal web browser will send some special headers when requesting a
file that it has received previously - I think they are
IfModifiedSince and ETag. Based on the values of these most web
servers will return a 304 Not Modified header (if the file has not
changed) and the browser will load the file from cache, or they will
send a 200 OK (if the file HAS changed)  header and the contents of
the file.

I've got a hunch that Outlook won't even ask the server if the file
has changed when you open it a second time.

Try to get the email content to open in Firefox and use the
LiveHTTPHeaders plug in to watch the HTTP stuff. Check if the image
caches in Firefox, if it does try to send a 200 instead of 304 (not
sure if CF ever sends 304's). If its not caching in Firefox, but it is
in Outlook - good luck to you because I think you're up the creek.

-- 
Mark Stanton 
Gruden Pty Ltd 
http://www.gruden.com

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