The asynchronous nature of email means that students and mentors can communicate despite very different schedules without intruding on one another. There are other qualities to email that assist in mentoring. These include the fact that thoughtful, deliberate communication is much easier because time can be taken to edit a message and have it say just what is wanted. This also makes it more efficient. On the web site there is some advice about using electronic communication including re-reading what has been written to be sure it has the tone intended.
Email is a very flat form of communication.
I can't see if you are smiling or frowning or whatever. One of our mentors wrote that the form of communication used on MentorNet made her work harder on being clear about what she intended to say. Taking part in mentoring forces you to be more precise. Maybe it will help people's writing. Really effective mentoring goes way beyond giving people the basic information they may not have.