The Inbox Preview Code Analysis feature offers a detailed line-by-line review method for you to check the HTML of your test email and its performance with different email clients and mobile devices. 

The most important point to take from the HTML code analysis is the possible negative impact on the rendering of your emails across email clients and mobile devices due to the HTML errors. An October, 2016 study from Adobe revealed that 22% of respondents say their biggest annoyance with email marketing is with emails that are not properly optimized for mobile. When email does not render correctly on an email client or mobile device, some subscribers choose to delete the email without reading it or mark it as spam. 

An HTML error won’t, by itself, cause your email to be placed in the spam folder. A poorly rendered email that leads to a bad subscriber experience that may be ignored, deleted without opening or marked as spam can have a direct negative impact to your inbox placement.