In the Mass Effect 3 Datapad, messages appear as you progress through Mass Effect 3. Some seem to appear just from progressing through the main quest line, but others seem to depend on sidequests, and possibly other things. What do I have to do to get all of the messages?
This would require reverse engineering the source of these messages; they could either be stored in the application, stored in the Mass Effect 3 client or been sent from their server. But that could be considered illegal to some extent, so the best alternative is either by completing the whole game or by collaboratively collecting a full list of them into a Wiki. I would guess that if you can find others with a Datapad, it could be interesting to compose a Community Wiki here.
Completion of the game in this case doesn't mean to just finish the game, but you would have to do all possible (inter)actions under all conditions to trigger the various messages. This would require quite some replays to get all messages, depending on the amount of messages there are...