Mail+ for Outlook - Outlook Email allows you to securely access email and calendar from Microsoft Outlook Exchange. With it, you can easily read and compose Outlook emails and manage your calendar directly from your iOS device. No more using the internet browser to access your Outlook account
CloudMagic for iPhone/iPad - CloudMagic is a powerful email client for iPhone, iPad and iPod touch. The app combines all your email accounts into a unified inbox, allowing you to seamlessly browse all the received messages. This also means that you can quickly search for a particular email across all your accounts
Molto - Molto (formerly known as Incredimail) is an email client specially designed to take advantage of the iPad's touchscreen and large screen real estate. Its user-friendly interface allows you to see more emails at a glance, quickly go through messages and keep your inbox organized
Sparrow is an iPhone mail client designed with love to provide you with an efficient and pleasant mailing experience.
With its pane navigation, its new threading system and many new features, you will never look back.
Warning: Sparrow does not support POP accounts and does not do Push notification yet