This has come up several times in the forum threads, and been heavily discussed. It comes down to authors and replying to emails is sort of like that song - some authors do and some authors don't.
Despite the other opinions, Ernest's is the most spot on. I have a hard time imagining any amount of email from a story being so overwhelming that I couldn't personally respond to each, especially for most of us, it's our only real payment—responses from our readers—something traditionally published authors almost never receive.
I not only respond to each email, but I tend to go on at length—so much so that many start out by saying "You don't need to respond … but I just wanted to say …"
However, many authors simply want to be left alone, while others only want to respond to the 'right kinds' of responses.
Essentially, over time you slowly learn how each author responds to feedback. Those that are eager for it, feel free to offer any suggestions, critiques of praise you want. For those slower to respond, or who say 'That's the story and that's it', you generally don't offer suggestions, whereas for those who don't respond at all, you simply don't waste your time with.
It would be nice if there was some SOL table where you could determine whether authors desire feedback or what kinds, but it's really hit or miss.
As for Switch's "I can't remember the details of my stories", I frequently have to refer to my story timelines, specifically to remember what happens in each specific chapter. After 16 published stories, it sometimes gets confusing remember who said what in which chapter. 'D