I have been using Adobe Experience Design (XD) for quite some time and it's amazing. It's meant for people who have never used photoshop or any other designing tools.

XD lets you easily create multiple screens, connect them and prototype for multiple devices.

We're using it right now for designing/prototyping our new newsletters.

Try it out.

Best for low res mockups in my opinion is (and will for ever be) pencil and paper!

And as for high res mockups, if you're a Mac user, Sketch is a really powerful prototyping tool that is pretty easy to learn.

Mostly Balsamiq Mockup and a pen / paper.