Sparrow is an iPhone mail client designed with love to provide you with an efficient and pleasant mailing experience. With its panel navigation, its new threading system and many new features, you'll never look back.
Sparrow for iPhone makes extensive use of swipes to help power users navigate the app quickly, a unified inbox, and top-notch Gmail support.
The app is missing some features considered essential to any email app. By far the most important is push notifications. Sparrow has a special page explaining why the app doesn't have push, along with a notification list where users can sign up to receive an email if push notifications are added to the app. Also missing is support for POP email accounts -- Sparrow only works with IMAP accounts like Gmail, iCloud, AOL, or any custom IMAP account.
Aside from those limitations, the app has been well received, with positive reviews from MacStories, The Verge, and The Next Web.