I've gone for a really clear and crisp look, taking inspiration from the colours of the gMail logo.
This is also my first foray into using Sketch, which I've found to be bloody brilliant for UI prototyping and layouts.
I've done a desktop/tablet layout, where the main interface has been whittled down to only the really important stuff - no guff. The mobile version is even cleaner, focusing solely on reading and writing e-mails. Designed around an iPhone, pressure presses are used to bring up context menus.