Mailchimp Integration?

I schedule emails through Mailchimp, and use them to create posts through Postie. The formatting is far from clean, with lines, boxes, and a bunch of distracting things that don’t fit with the style of the site. I purchased and installed the Clean HTML add on, but it didn’t work on my most recent test. Is there something I’m doing wrong? At a minimum, I want to posts to show up as the emails do in my regular inbox, without the outlines.