Responsive Rendering Oddity in Apple Mail, iOS 8.4, iPhone 6
It turns out that it's a temporary bug. Not sure of the cause, but closing and reopening Mail fixes it.
I’m seeing issues with responsive layouts in Apple Mail on iOS 8.4 on an iPhone 6 across all of the Code School emails that I’ve tested. The emails render as expected in the Mailbox app and the Litmus tests.