Email, as we know it today, is not instantaneous because it adheres to physical principles. Messages are essentially duplicated as they traverse multiple relays, sometimes passing through several nodes before reaching the intended recipient. To draw a parallel, think of the postal parcel delivery system, but with a crucial difference: the message is cloned multiple times, creating copies at various points along the route. To add to the trouble, there's spam filtering on both sending and receiving side and often in between.
In contrast, within a pull-based messaging system, the message becomes instantly accessible upon upload (creation), and the sender (author) retains full control over the message even after it has been uploaded. This functionality makes actions like "undo send" and "update" straightforward to implement, without requiring any UI trickery.