Twice in the past week people have complained that I did not respond to their email. In the first case, they sent the missive to the wrong address, waited two weeks, and then wondered aloud to me if I was really busy and was unable to keep up with email. Well, yes to the really busy part, but no to the unable to keep up with email. I cannot respond to email that I have not received.
In the second case, the sender was upset because they had sent an email in the morning and I had not responded by mid-afternoon. It was a day when I was out of the office with a client, and had back to back meetings throughout the day. I was not checking my email.
The "always connected" Internet culture has created a false sense of connectivity for not just email, but also for things like texting. I travel in a lot of rural areas, where cell towers may be few and far between, so I don't necessarily receive texts a few seconds after someone sends it.
Email and texting are asynchronous services; you send the message, and it may or may not reach its destination in any given time. There seems to be a rising resistance to using the phone for business communications, even though the phone is a synchronous communications medium--if I answer the phone and are speaking to you, you know with certainty that I have received the message.
There is a certain irony at work in our culture when we all have "smart phones" but don't actually use the phone part.