Voltage Security has a secure email client for both iOS and Android devices that mimic the same user interface of the device’s email apps. It is easy for IT to manage and scale without a lot of hassle for email message storage and key management. It is easy for business users to adopt without cumbersome certificates and web links.

Requires iOS v 5.1 or Android
Price: Free to download , requires Voltage SecureMail platform
Voltage Security
http://voltage.com/securemailmobile