It's Windows only, but they're not going after Outlook. And it's not actively going after webmail based users.
So that leaves a small minority of users? Of the people I know who use email on Windows exclusively they either user Outlook or webmail. There's the odd user on Thunderbird, but that's about it.
If I'm missing something I'd like to be educated..
Since Thunderbird development seemed to have slowed to a crawl, I thought I'd try a product with a company standing behind it:
- A few months ago I tried PostBox. The e-mail piece worked great but it crashed as soon as I tried to use the Lightning calendar plugin. Their response was that Lightning isn't provided by them so I should take it up with the developers of Lightning. Having the problem dumped back on my lap was what I was trying to avoid by paying for a product that a company stands behind (fortunately I was still in the free trial period).
- Then I tried Mailbird. It's a fun-feeling product which supports Google calendar integration (and other 3rd party integrations) but it couldn't handle event invitations from Outlook, which makes it something of a non-starter for anyone looking to use it in most business settings. They were responsive to my inquiries, however and said that feature should be available in version 2.0 so I'll probably give it another try then.
It seems to be aimed at increasing productivity for users with multiple accounts while remaining usable. I think they're mainly aiming at businesses with this, something like small business sales perhaps.