In the specific case of hotmail, everything from the top of the email (the doctype declaration) to the first table tag in our email was overwritten--eliminating our stylesheet declarations as well as any <body> tag modifications we had in place. To add insult to the injury, Hotmail rewrites all the incoming image SRC"*" and HREF="*" tags to point to their own caching servers. But beware: the caching servers time out, so if a user keeps an email message open too long (likely, if you send a lot of content), the HTML redirects that Hotmail substituted for your URLs start decaying.
I would be interested in your tests on Yahoo. From earlier casual observations, I suspect the results will be the same, but you are obviously being more careful in your analysis.
Strangely, if you forward the message from Hotmail to some other ISP or if you download your Hotmail messages with Outlook Express, nothing gets rewritten--the original message arrives in the remote inbox as though there had never been a badly mangled version sent from Hotmail.
Wow. I think you will *not* find this with Yahoo. In specific, I think that Yahoo will forward the message with the SRC"*" and HREF="*" tags in their re-written states, but I could be wrong.
--Paul Hoffman, Director --Internet Mail Consortium
