First, the good news – it’s very likely that you don’t have to change a thing. As the iPad Mini has the same display dimensions as the iPad 1 and 2 (1024px x 768px), there’s no need to adjust your existing media queries if you’re already targeting these devices… Unless you’re using device-pixel-ratio for really selective targeting (which we generally don’t recommend).
But should you be targeting the iPad when creating a responsive email design? We’d like to argue, no – the 768px device width in portrait mode is likely enough to comfortably view most email designs in their regular, non-mobile state. If anything, iPad shares more in common with desktop clients like Apple Mail, than mobile clients from an email design perspective. So it’s generally not worth narrowing down your designs as you would for iPhone or Android handsets.
For more design guidelines for the iPad Mini (and iPads in general), I recommend reading this post by Mobify. While it doesn’t specifically cover coding for email, it’s full of practical adaptive design tips and considerations that you can apply when designing with mobile devices in mind.