Have you ever sent an email (or left a voicemail message, for that matter) and never heard back from the person you contacted? Then, when you finally reached them a few weeks later, they said, "Oh, yeah, I was just busy and didn't have a chance to respond."
Now, I'm not referring to the spam or unsolicited emails we receive from people we don't know. I'm talking about emails from people who have a reason to communicate with us such as family, friends, and colleagues. I believe we do have time to respond to these folks.