Philosophy of the two minute rule
I always understood the whole idea behind the two minute rule was that if it was something you considered doing and it took you longer to write it down on your list than actually doing it, then you would be better of doing it.
That principle applies for that e-mail you need to send to a client because he's waiting for it. It also applies to that e-mail you're sending to an old friend ("how have you been") because the thaught crosses your mind that it has been a long time you heard from him and it takes you longer to write it down on your S/M list than to write the mail iteself.
Strictly speaking you then applied the two minute rule to a S/M item. Nothing wrong with it I think, just don't make a habbit of filling your day with that kind of items because it might get in the way of your productivity...