UPDATE 2016-01-03:There is a mail client called Nylas N1
Over the last maybe year or so I have been plagued with a problem, I use as my mail client under Linux, and for the most part, it works okay, it does it's job. But it is rather heavy, somewhat discontinued, and I just would like a better-looking (cleaner) email client.
Initially my sights turned to as an alternative, looks good, works great, and seemed to have most of the features I wanted. However it only has IMAP support and as such won't work for my hotmail account or my work email (both still use POP3, although that may change in the future).
This led me to keep thinking about the alternatives. And then I thought "wouldn't it be great if I could access all my accounts from the web? keep them all in one place rather than on each separate device?". My initial thought was just making my own mail client (like what I did with when I wasn't happy with the existing offering), but then I did a little bit of research and found an awesome crowd-funded idea for an open source web-based email client called .
It looks sexy, web-based, can be run on a server (meaning I can keep all of my mail in one place, along with what POP3 messages have been opened, so I would only have to open them in one place!), and just be kept open as another browser tab. The HTML editor but with the rate the project is accelerating, it shouldn't be too long before it is implemented.
I will be giving it a shot tomorrow and install the beta on my desktop, and if I like it I'll probably run it on my server so that I can have the one installation of it everywhere I go.