Its an asynchronous medium, like the postal service. Its not a conversation on the telephone. Replies are not expected to be instant. Anybody that calls you up 1 hour later to find out why you've not answered is clearly "doing it wrong"®. Use the correct tool for the job.
The guy could have stopped spamming people and just answered his emails once a month instead for the same effect.
People that claim email is a bind or that they are a slave to it should have it removed and see how they prefer life.