Enforce Strong Security
While this may seem obvious, its also a basic foundation for more advance security. Require alphanumeric passcodes and encryption, and limit failed attempts. Also, consider master data management systems for provisioning and management, with the recognition that device data security may not be significantly enhanced by the MDM software.
Reduce Stored Data
Put a limit of a few days on email storage and restrict attachment downloads. More importantly, provide mobile access only to employees who actually need it, making sure those employees are made aware to keep highly sensitive data off email accessed through mobile devices.