Which was and is a lame assumption. Stuff happens. SMTP wouldn't even be phased by this; it would just pick up where it left off.
I've seen far too many applications fail in bizzare ways because people make unrealistic assumptions like "X will ALWAYS be there". Sure it's highly unlikely, but when you have multiple things making the same dumb assumptions, on the inevitable day when multiple things that need X and X is suddenly no longer there then you start to get cascading effects of Y that relied on something that relied on X not being there when it is assumed that it would always be there so now Y fails, and then something dependent in the same way on Y unexpectedly fails and so on.
One should never assume that anything will "always" be available. That's an incredibly unrealistic assumption; and the more interconnected things become, the chances of these really nasty dependency chains/cascade failures skyrocket - leading to far worse outages and longer recovery times.
