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
OWA for iPad - OWA is a mobile client for Outlook Web App that offers business users access to their emails, calendars and contacts from a native iOS app.
With it, you can organize emails, schedule meetings, view calendars, find contacts using voice, remotely wipe corporate data, etc