Today’s life lesson: never use email where a phone call will do. It’s a tough life lesson to learn. Email has become SO easy these days (and yes, I’m including facebook and linkedin messages as email).
More specifically then the general rule, decide whether you want your message to be remembered or never be forgotten. If you want your message remembered, pick up the telephone. You’re mentor, perspective employer, customer, etc… will remember the gesture months from now when you run in to him/her again. If you need your message never to be forgotten (e.g. you may later need to prove you wrote it), then (and only then) should you send an email where a phone call will do. Even then a follow up phone call is not a bad idea if you want your message remembered as well.