It is multi-platform. I've used it in Linux, Mac, and Windows. I can use up to 4 accounts for free, and it has built-in and free limited visualization tracking and signatures.

It's the best overall tool if you have multiple inboxes, as you can get a quick evaluation of which to tackle first. It's clear what emails have not been read with a universal inbox and individual boxes tool. I like the simple design.

The design interface is very nice, and it's easy to set up.