I've now broken Outlook 2003 trying to get my email to work in Outlook 2007! Boo-hoo
Any advice on getting this to work? What should I definately NOT do?
Unfortunately Outlook doesn't reveal much about how it works, but I can tell you we have thousands of campaigns going through Outlook filters fine, so it doesn't always get caught.
Your best approach is to cut out half the content, and send the rest - that way you can try to narrow down where the problem content is, and try switching it up.
And to add to Mat's comment, I've seen it even be picky with your image filenames so if you have a file name that contains even a part of a word that could be construed as sketchy, change it. Ie. november-ad.jpg, I'd change it to not contain the word ad (just as an example).