K-9 Mail brings support for multiple e-mail addresses and high-tech features, but it's not easy to set up. It leaves Microsoft users out in the cold for the most part, too. If you know how to manually set up an e-mail client, this app will delight you. If you don't, you might want to stick with the default app on your smartphone or tablet.
This app claims to not support many Hotmail or Outlook e-mail addresses. It will require an app-specific password if you want to sync with Gmail, too. If you're having trouble, you have to go the less automatic setup route, which might scare you off. If you've set up Outlook, Thunderbird, or a similar client, it won't freak you out too much. Plus, K-9 Mail has a very active coding community that is willing to help you troubleshoot any errors. Once you've gone through all that, the app's support for multiple addresses is really wonderful. It includes color coding, a very attractive unified inbox and other goodies. The app, itself, is streamlined, which is really pleasing to the eye. It features cool ways to scroll your mail through gesture controls and even volume-rocker scrolling. There are some useful privacy and notification settings, including a great "Do Not Disturb mode." It only shows 25 e-mails at a time. That isn't much, but it may help you deal with inbox clutter, head on.
In the case of this app, great power comes with a little know-how. If this isn't your first e-mail client rodeo, you won't mind the setup and you'll be rewarded with a truly powerful app. In fact, K-9 Mail offers so many cool features that it's worth learning how to set it up just to cash in on them all, anyway.