It can’t simply be that you’re so busy, can it?
Are you simply too important?
Maybe you just forgot, and you had meant to respond.
I’m sure for many people the volume of email today is breathtaking and completely unmanageable, but not for everyone.
Could it be, that you’re simply inefficient, and replying when you’ve read the message is the solution?
Whatever the case, please make sure you reply, cos it’s rather rude not to