Hi Ted - Cloudfront is the CDN we use to deliver all of your page assets. It’s Amazon’s CDN, which is one of the largest (and also most reliable) CDN services available.
There definitely have and will be some malicious files that have been hosted with them, but that’s the same for any major CDN, since they’re all hosting millions, if not billions of files, but there’s no way that should be triggering a content filter, since blocking cloudfront would mean blocking a massive number of legitimate websites and hosted files.
Can you try sending this URL to your friend to see if it’s blocked: http://unbounce.quinnomori.com/content-filter-test/
That’s one of our templates with no changes, so I can confirm that there won’t be any actually malicious files linked–there is the off chance that you have a corrupted image or something else that has been uploaded that’s setting off their filter for some reason, but if that’s the case, that test page should be fine.