iPhone Native Mail App mail rendering
Have the same style, font and text size of the native mail app.
I say this because many of my emails displays Times New Roman like font and signatures from senders doesn't display correctly like in the native app.
I would simply send them a rendering issue report. They usually fix this very fast.
It helps of the basic interface is similar to ios native email all
I have the same issue. Sometimes the font type is wrong, sometimes the font size/line height.