In my experience, gMail has pretty strict content filters. I would suggest testing your message in the following way:
1. Send a blank email - if gMail puts it in the SPAM folder, you know it's something to do with your header, subject line, or "from name".
2. If the blank message passes, you know it has something to do with your message content.
HTML messages are usually not blocked entirely - the images are more likely to be blocked automatically (in Outlook, Hotmail, and gMail).
More importantly - focus on the ISPs that are most important to your business. Corporate SPAM filters will be the hardest to crack, but if your business is mostly coming from Hotmail or Yahoo, you're already doing pretty well.
Also - are you Sender ID compliant? That could have a lot to do with your issues...
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