If you get the message "502 Bad Gateway" or "404ish Syntax Error Invalid Charactor, 502 Bad Gateway" when launching your email, check the email draft for an animated GIF. Animated GIFs are not currently compatible with VerticalResponse emails.
An Animated GIF is an image that moves (you might not have known they actually have a real name). Once you remove the animated GIF, you should be able to launch your email without being held up by errors.
If you like the image featured in the animated gif and want to use it in your email, please create a screenshot of the image and save it as a jpg.