If you're looking for fast, smooth, full-feature third party email clients for WP 8/8.1, you're out of luck. Metromail is full-featured, but like all the third party email clients on the platform, is rather slow.
The fastest app is, of course, the basic mail app that comes with WP 8/8.1, but it's also severely lacking in features.
Unfortunately, this is rather systemic of the platform. Try to find a calendar app that isn't just a wrapper for the built in calendar, as another example of either:
a) Poor platform apis
b) Poor developer skills
c) Some combination of the two
..that are available for WP.
At the moment, I make do with the built in mail app, after spending a ton of time on the actual email services working on a gazillion filters so as to keep the inbox manageable. I do miss things like "tabs" and other basic organizational concepts email apps on other platforms have had for...a while now.
And, yes, Metromail has this type of stuff, but it is so freakin' slow. I'm not sure if I can really fault them, as I don't believe I've encountered any third party email app (not that there are many) for WP 8/8.1 that isn't super slow.
There is a "new" app that is supposedly in the works that at least looks pretty/like something you might be able to find on iOS or Android. No idea if they've managed to make an imap client that is actually fast for the platform. But, you can keep an eye on it for its eventual beta release at:
http://www.maestroapp.com/ which their latest update claims will come out this month.