We like Boxer when it initially launched because it's an email app that includes all kinds of features, including integrated to-dos, quick archiving and Dropbox/Evernote integration. Now it also comes packed with a calendar and contact manager.
The Android version of Boxer comes a la carte, with a separate Calendar app that integrates with the email app, while the iOS version has the calendar rolled into the email app itself. The calendar allows you to instantly check and share your availability right from the app. The iOS version gets a new contact manger as well, so you can manage contacts and email your favourites quickly. The app's free for one account, but you can unlock more via in-app purchase.