iOS: You have a lot of different options for email clients on the iPhone, and many of them are very good. If you're looking for an email app that does things a little differently, Boxer manages to combine the best features of other apps into one single email client.
Boxer has a whole set of tools for dealing with emails. You can easily archive with a swipe, send instant auto responses, add emails to a to-do list and "like" incoming messages so people know you saw it. Boxer also links with Dropbox for file sharing, and syncs up with LinkedIn and Facebook to give your contacts profile cards. Everything happens with gestures, and the interface is intuitive to use.
Boxer supports all the major email providers, including Exchange, Gmail, Yahoo Mail, iCloud, AOL and Outlook. If you like the idea of some of email apps like Mailbox, but you need a little more support for email power user features, Boxer is worth a look. Boxer is currently free for the first 100,000 downloads before it bumps up to $5.49.
Boxer ($5.49) [iTunes App Store]