forward formatting messed up

Hi guys,

I'm running out of ideas. I've been trying to figure this one out without success for weeks now. Here's my problem.

I've set up a custom newsletter template and I can receive the newsletter without any formatting problems.
However, when I try to forward the newsletter (via my outlook 2007), the formatting is switched to plain text which changes the newsletter to a bunch of gibberish.

I've tried changing just about everything in outlook related to formatting...

Tools > Options > Preferences (tab) > E-mail Options... > (when forwarding a message) Include original message
Tools > Options > Mail Format (tab) > Compose in this message format: HTML

and many other settings... but the problems persists.

I know most people would think this is an outlook problem, which I originally thought as well... but the reason why I'm considering that it may not be is because other newsletters that I get form third parties (non-drupal generated) don't seem to cause the same problem. In other words, my drupal newsletter is the only one giving me a hard time (that switched to plain text).

So I'm wondering if anyone else ran into the same problem.

attachments:
mail.inc (is from \includes folder)
my custom newsletter template

Thanks in advance for any tips!