Dont worry, its because of HOTMAIL over active spam filtering, it may only happen for you, as you are using mailchimp, mails go from their reputed IP and it doesnt matter if its text or HTML, if you are all done with DKIM and SPF then everything is fine, dont worry, it must be you among the very few who receive it spam with any provider, and your deliverability will improve as people start opening your emails
If you want to check, try to another hotmail on different IP/browser, things should be fine in there.
Thanks