E-mails are the most efficient way to communicate with people if the culture of the company makes it so that is the main form of getting into contact with someone. If the person that you are trying to getting into contact with does not regularly check their email than it becomes calling the person. This relates a little to emotional intelligence because if you are rightly annoyed that this person does not use the main form of communication you can not show this or the task that you are trying to get done will be further delayed because you are more likely to work harder for someone you like and respect.