The Apple Mail app is the default email application for iOS. The Mobile Encryption for iOS System Requirements state that it only supports the Apple Mail app for encryption and decryption of email.
However, even though Outlook for iOS is not officially supported, it is often possible to decrypt messages received with Outlook for iOS.
Note that even if you use Outlook to receive encrypted messages, Mobile Encryption for iOS sends only with the Apple Mail app. Therefore you will need to configure the Apple Mail app for sending.
The steps for decrypting a message recieved with Outlook for iOS are as follows:
This launches Mobile Encryption for iOS and decrypts the message.