So, I updraded my iPad to iOS 9.3.5 yesterday and I noticed that some emails (from various senders) showed a gray 1px border below the subject line while others did not. Upon further investigation, I determined that if your <body> tag contains a bgcolor attribute, then the border is shown. If you ommit the attribute, then the border is not present.

I thought this might help OCD email geeks like myself.