One thing I did notice (once you check the
solution in ManDrill) is that my email notifications (which were formatted in html templates) coming from Formidable pro were getting *garbled* as in it was a bunch of css and html instead of a clean, name:, email:, comments: notification that someone filled out a form on my site.
I saw that you can have the notification email from Formidable sent in plain text and it works fine now. So if you have any plugins with the same issue see if you can set them to plain text!