We're repeating the process I stated above using the other axis of the toaster. Here, you can see I've already gone ahead and connected the edges and am chamfering them out. We do this such that we create a little square in the corners. See that? It tells us that when this is sub-divided, the effect of subdivision will be constrained to just the edges and corners- the whole box won't be all bubble-like. The reason we're going for squares in the corners is because it makes sure our smoothed edges are uniform around the toaster. A rectangle would yield different results. Feel free to experiment!
Finally, I've connected and chamfered the edges along the third and final axis. You'll notice that we now have every edge of the box supported with two more edges that keep it from being overly smoothed.
Here I've applied a turbo-smooth to the model to give us an idea of what it's going to look like as a finished product. We can see the fruits of our labor thus far in that the box has constrained smoothing and is looking like a mighty fine box so far. I've given the 'T-smooth' modifier one iteration in the viewports to keep the viewport speed up, and two iterations at render-time to make it look smoother when we want extra quality. Render it if you like by hitting F9.
Refining Our Initial Model
So we've gotten a lot of the basics down, but the project isn't quite complete yet. We don't have the bread slots and the pull-handle. It needs characteristics of a toaster. Let's bring this model to life!
To start refining the model, let's break this toaster into two halves- right down the middle! We use the same technique as I listed above of selecting the center vertices and connecting them. You can also use 3DSMax's own "ring" tool by selecting one of the edges in the set and clicking "ring" under the "selection" rollout. To find out more, hit F1 and type into the search bar "edit poly ring". As you get more and more edges, it would be wise to understand the "ring" and "loop" concepts.