This is true. However, email is explicitly not a reliable/guaranteed delivery system, so, despite the fact that is is really quite reliable, we keep that in min

This is true. However, email is explicitly not a reliable/guaranteed delivery system, so, despite the fact that is is really quite reliable, we keep that in mind when designing systems that make use of that channel of communication. SMS? Not so much. Idiots persist in thinking of it as reliable/guaranteed.