Due to the application design rules on the Apple iOS device platform, notifications work differently on those devices. This note explains how Apple iOS devices receive notifications.
As the DME client for Apple iOS is shut down every time you log out (due to iOS design rules), it is impossible to have it listen for push messages from DME, either using network push or SMS push while the client is not running. To overcome this problem, DME has removed the network push option for iOS devices, and makes use of the Apple APNS (Apple Push Notification Service) framework, which makes sure that every Apple iOS device in the world can be contacted via Apple APNS servers. The notifications are thus sent from the DME server, via Apple's servers, to the iOS device. The notification is presented to the user on the device, and he/she can then take some action, for instance launch DME.
As an alternative to the APNS solution, DME has implemented a form of SMS notification - the SMS received contains a URL on the form dme://<command>, which the user can press in order to open DME and start the action (for instance synchronize e-mail).
APNS is required if you want to receive push notifications on Apple iOS devices.