Prior to the release of the Symantec Mobile Encryption for iOS Mail Encryptor app, there was a separate Viewer app for iOS which did not require a separate license.
The Viewer app has been withdrawn but Mobile Encryption for iOS can be used to decrypt email without requiring a separate license.
If Mobile Encryption for iOS is used to encrypt email, it requires a separate license.
The Mail Encryptor app can be configured without encryption capabilities. To do this, install the Mail Encryptor app on the iOS device and enroll the user to a Consumer Policy that does not have the Enable Mobile Protection option enabled:
Users enrolled against a policy that does not have the Enable Mobile Protection option enabled will not be able to encrypt mail; only decrypt it. Such users do not require a separate license for mobile protection.
Note that all managed Encryption Management Server users or devices require a license.