IM and SMS are different ideas. SMS moves a message from one device to another. SMS also knows nothing about the receiving device or whether the message got the

IM and SMS are different ideas. SMS moves a message from one device to another. SMS also knows nothing about the receiving device or whether the message got there. IM (under which I include iMessage) is user-to-user, not device-to-device. It can know about the recipient and whether messages are received. Each of these things has advantages. SMS works because the phone network is always tracking a device. It is very addressable. The receiving device is mobile, rarely changes, and is singular. IM has a notion of sessions. The user signs off and on. It can travel over any IP connection. The device on which the user is addressable changes a lot. There may be multiple devices, making the definition of “delivered” a bit less deterministic. Conflating these two makes for a confusing mental model for the user, and for failures like this.